Have you replaced the shooter rod rubber?
I know on my star trek when I added the shooter housing light mod, I Had to add a little paper towel wad into the end of the shooter rod rubber due to the light housing taking away a quarter inch out from the cab. This will help with giving the ball more force out of the shooter lane.

Replace the sleeve and spring.

First off, is the plunger tip hitting the ball dead center? Most that come thru the shop need adjusted. If it's hitting too high or low that takes a LOT of force off the ball.
Also, wrong spring on there possibly? Put a red one on there....

What about the sleeve?

http://www.pinballlife.com/index.php?p=product&id=92

They sell different tension springs, get one with higher tension and it should pop faster. Typically a game where the ball has a long plunge or goes up a ramp have higher tension springs. Try a game like High Speed and see the spring type.

10-148-1 .035" standard silver spring is the correct spring for Space Shuttle. It's likely that something else is wrong (shooter alignment, etc.) or possibly you have the game jacked way up and that is a factor? From there I would move to the next higher tension spring, 10-148-3 Shooter spring, blue, .038"

If everything else is in alignment and you still have problems I would upgrade the spring. You might need to experiment but try purple or red from pinball life since a standard spring doesn't sound like its giving the plunge you want.

Red
http://www.pinballlife.com/index.php?p=product&id=126

Purple
http://www.pinballlife.com/index.php?p=product&id=2920

What is the best way to realign the shooter rod to hit the center of the ball? I have the same issue on my TAXI and there doesn't seem a way to adjust the shooter rod angle?

First, make sure the playfield sits all the way down in the lockbar receiver (the brackets can sometimes be bent) and that the rest for the ball in shooter lane is straight. Next, slightly loosen the three mounting screws on the inside of the cabinet for the plunger housing. The whole assembly can be moved to align the plunger tip with the ball. Keep in mind that dead center might not be the best position. Some plungers work better if the tip is center and slightly above the ball to give it some "english". On my Pinbot, the ball actually traveled better with slight right english.

plunger_adjust_2 (resized).jpg

This was my first question. What color spring did you buy. Followed by alignment, and then is the rubber on the end of it all the way down (a gap between the rubber and the metal of the shaft of the shooter rod is like a shock absorber there).

You can also bend the 2 prongs a bit (where the ball sits against) towards the shooter tip to get more power. I once got a (barn project) game where it had the proper shooter, but the tip just didn't quite reach the ball. Rather than mount the plunger rubber with a spacer for added length, I bent the prongs. Worked great for me.

You probably saw this at the links provided above, but:

Spring tension from lowest to highest:
Blue (266-5001-05)
Green (266-5001-04)
Standard (10-148-1)
Orange (266-5001-07)
Purple (266-5001)
Red (10-148-2)

Well on my Space Shuttle I've never replaced the spring, but I had no issues when I got the game so I left it as is. When I got my High Speed recently, the plunger couldn't make the ball up the ramp all the way so that's when I started looking into it and realized the spring was either old or incorrect. I got the correct "red" spring as noted above and it shoots great now. I've never adjusted any of my alignments (and to be honest didn't even think of it until this thread - good to know!) and just replaced tips pushing the rubber all the way down (which I don't know if that's "correct" but it's what I prefer and it works for me). I think as someone above said, the correct spring for Space Shuttle isn't the red one, but I can't see it doing any harm if he's not getting the power he needs to work his way down the tension list.

Yep. Basically, whenever there would be metal moving against metal, there needs to be something in between.