Try a couple of tricks:
a) Dump the render files you've already made of other parts of your sequences;
b) Move everything in your sequence to the right (using the double arrow pointing right), then moving everything back into place;
c) Set another Scratch disk for your render files;
d) Cut the portion in question out, paste it into another sequence, render it, then put it back into the main sequence;
d) Checking your render settings.
Sometimes FCP is fooled into thinking a piece is already rendered when it's not. I'm not sure that's what's happening in your case (too little info), but the above tricks can help clean the slate and force the computer to render.