My Alien Star was doing this on Saturday. A quick lift of the playfield and slight bending of the switch resolved the issue.
It looks like Gladiators is a DMD, so you'll have some form of switch test in the menu. Get into the switch test and push down on the switch with your finger and see if it registers. If it doesn't then you'll want to lift the playfield and really make sure the switch is closing completely. If it still doesn't work, chances are a wire has come loose.
If it does work with your finger, then put a ball in the saucer and see if it registers. Chances are that it won't. In that case, slightly bend the switch until it registers when the ball is in the saucer.
The entire point is that sometimes they will register with your finger, but the ball when rolling over / sitting on the switch doesn't push the switch down far enough and the metal arm needs bent.