amp problem?

i went back in, and added another set of rca's for my 4 channel set-up. before it was set on 1 up front, on 2 channels, and everything worked fine, but when i put everything back up, and tested it out for a final spin, my left front speaker wouldn't play. i put it back on 2 channel (through front ones), everything worked fine, every speaker played well. so the connection is good, and everything is hooked up right, since the front left is playing on a different set up. it the process of mounting an amp, i've dropped it, when it was on its side already, so shouldn't be anything serious, but i wonder if i've damaged something. it's only 2-4 weeks old, so might still be able to return it, and get another one, or go through mtx costumer service. how is it by the way?

turns out the cheap rca's that i got in my wiring kit. one head just fell off after i was playing around.