Grammars include begin/end rules, when inside those rules the only thing that is active is the rules listed in it's patterns list. So if your trying to include something inside there it's not going to work.
The other possibility is that your trying to include something inside a match rule, there your match rule will win because it is the highest in the language file. However that is only if the matches start at the same place. If a match in the main grammar matches in an earlier column your match is never even seen.
For the first problem there is a solution coming at some point; but it won't be very soon. (Search the archives for scope injection) For the second nothing can really be done except duplicating that rule yourself.