verilog-module instantiation

Everytime I've done that, it's because I forgot to drive something into the module, it became unknown and propogated around. You said you're driving everything, but maybe worth a double-check.

I think modelsim has a tracex capability, where you can trace an x back to where it came from. I only used it once and struggled to figure it out(I have very little modelsim experience), so maybe someone else can post more details?