Clear RCX Hardware Issues

I may have mentioned earlier that I’m in a mindstorms-robotics class at school. Well, this past week my partner and I ran into issues where the (Robolab) 2.9 “0 Rot.” block made the bot go forward, then back when it’s code told it to 0 the rot. sensor on port 3, then go forward for 0.5 sec. and then stop. The same prgm (program) just without the “0 Rot.” block worked perfectly, and now this morning when I tried both on one of my RCX bricks they visibly did the right thing. Do any of you know why this may be aside from the hardware itself failing?