sounds like a
wago connector
terminal looks something like this, right?
the ones on your ballast maybe styled differently, but the bar drops to lock the wire into place, if i am not mistaken.
1. is your wire stripped sufficiently to go into the wago connector? the length of the exposed conductor should be aprox 3/8" of an inch
2. is the wire on your cord too big to fit into the connector? see how on the image the connected wire is shoved all the way into the back of the connector... you have to push it in
hard.
3. kinda obvious, but the bar must me unlocked (up) before you try and put the wire in there
4. is your wire too small for the bar to lock down on?
5. and if your wire is striiped, is the end of it frayed? if so you have to retwist the strands together to make them tight again,. even one loose strand is enough to keep the wire from seating in the connector properly.....
just a few pointers..