The first thing the replay button does is activate the Replay Button (V) relay. If yours isn't doing that and you have replays on the counter, diagnose why with this circuit. Do you know how to use Alligator Jumper wires?
http://www.pinrepair.com/em/index3.htm#features

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That sounds like a playfield switch is making contact when it should not be.

Hi TimC
I see in Your video as You manually reset the Control-Bank - then toggle-on and the ZB-First-Ball-Relay plunges and the QB-Game-Over-Relay plunges --- both is needed and done in Your pin, good.

There is one more thing You may try --- (the pin is toggled-off): Stepping the Player-Unit to Home-Position and some 16, 17 steps further - please read here https://pinside.com/pinball/forum/topic/magnotron-not-resetting-fully#post-3979911 starting from "The secrets of the Player Unit" ... till the end of the text. See (there) in the second JPG the (fifth cam) "Home-Position" --- it is much better to step some steps BEYOND the Home-Position - then manually reset the Control-Bank - then toggle-on and then try to start a game.
If still "No luck": Try a little cheating - use a wooden stick and press / move the armature on the S-Start-Relay. Greetings Rolf

I'm not sure why it would, but for now I would manually press the start relay and get it through its start sequence. Then when it starts scoring points, I would find which switches award that point increment and check them all to figure out which is stuck on. If that solves the start problem, then great.

If not, I would check the switches on your replay unit (assuming you have checked and confirmed that you have credits on the wheel) there are switches there that if they are not making contact it will not allow you to begin a game.

The switches in EMs do get dirt in the contacts periodically. Sounds like that is what happened here.

Dirty contacts in several switches. If it it starts acting strange again, adjust and clean as needed. It's not a bad idea to go through every relay and make sure they are in spec.

Sorry but I have to disagree.

As to the first, properly adjusted switches will almost always stay clean and working for years. To the second, bank and motor switches are prone to being jarred by untrained fingers and accidentally moved out of adjustment, causing new problems. Even normal relay switches can suffer from attention from untrained fingers.

With that said, @d-gottlieb, if you have a maintenance routine that works for you then I wouldn't want to tell you what to do, but for others with less experience, I highly recommend only fixing what's broken.

Yes, I agree with your ideology. However, I continue to run across EMs that are adjusted and look ok, but the contacts are just plain covered in crud. Like I have said before, to own an EM is to work on it as well. Once it is adjusted and cleaned up, as far as the contacts are concerned, it should last for years without further attention.

If you want your game to go up in flames, then contact cleaner will help.

This is a great resource:
http://www.pinwiki.com/wiki/

Hi TimC +
I often read in pinside about "using a flexstone" for cleaning contact-points. Actually - I never have seen such --- here in Switzerland I / we use very fine sandpaper --- it is so fine and its called "car polishing cloth" - as some people use it to "freshen up old car paint" - actually they "sandpaper away" a bit of the car paint ...
I often read "a stripe of rough cardbord - rough card" is enough for cleaning the contact points.

I like the "precious Self-Cleaning of contacts" --- well-adjusted switches. I believe in the 50% rule --- want to see here https://pinside.com/pinball/forum/topic/grand-prix-reset-issue/page/2#post-3831198 and then follow the link. Greetings Rolf
P.S.: Gottlieb pins / switches have very little travel of the switchblades --- in Gottlieb pins I usually am happy to adjust the switch so it works and take everything from 30 to 70% ...

This ^^^^^^^^^^ x 100!

If you are new to EM's I guarantee that if you go and clean/adjust every switch in game you will make things worse. As HowardR says it is much easier to fix what is broken! Why clean 100's of switches and riskj making things worse when you actually only need to find the one switch that needs attention and deal with it.

I do appreciate that sometimes it may be necessary to go through all the switches if you don't have a schematic and have come to a halt in troubleshooting. However, usually enough info can be gleaned from schematics of the same era in order to point you in the right direction. There is lots of help on Pinside...I personally enjoy the challenge of trying to pinpoint the faulty switch and love to help others when I have the schematic available.

BTW my first game ever was Jack in The Box that I purchased back in 1983 - I remember using contact cleaner I mean why wouldn't you right!? Thankfully the machine didn't catch fire.... but I sure was lucky!

Oh my GAWD!!! when I think back on some of the things I subjected my first EM to ... luckily, unless you get a fire going, they're fairly forgiving.