There seems to be some weird cap, or a scaling issue that prevents any higher thrust strength value from working.
It works for small changes, such as a strength value of 1.0 or 2.0. Anything higher and it does not change your velocity.
It's currently set to something like 25 in the mobility - thrust blast effect group, so you'll have to lower that to 1 or 2 to see it work on most ships. It also cares about thrust to mass ratio, so make sure there's some thrust on the ship too.