Hello,
It depends on how many X-mismatches(exp=0/1 got=x) are there?
If they are huge,then check whether clock is propagating to that cell or not ; check whether cell gets initial reset or not. These are most possible reasons I have seen for getting X-mismatches.
In case it isn't clock/reset issue, you need to
1. dump the waveforms, load your design & waveforms into Verdi,etc tool.
2. check on which cycle cell captures X.
3. Backtrace your X-capturing cell in your design to find X-source.
4. Check whether it's due to some cell constraints.
If it's a single X-mismatch,then you may need to dive-in a bit deep to debug the issue. However, solving a single X-mismatch(exp-0 got-x) is quite easier than solving single 0/1(exp-0 got-1) mismatch.
Happy debugging .!
Thanks.
-S