Probably. Both of these should work, as long as you are setting the
priority high enough, so it seems likely that for some
reason the template in the importing/including stylesheet isn't
actually matching whatever it is that you think it should be matching.
Perhaps you've mis-spelt something in the match pattern or the
overriding template is more specific and doesn't actually match the
source XML that you think it does?

Posting the overridden and overriding templates, and the relevant
portion of the source XML, might give us more clues.