I never let excel guess at the range to filter. I always select the range I
want, then apply the filter.
Maybe you could try removing the filter, reselecting the range, reapplying the
filter and then filter once more.
If that doesn't help, you'll have to add more details -- what you're filtering
text (including length of text), dates, times
how you're filtering (contains, greater, equal...)
and how it fails.