You need to be in the capture for a minimum period of time for you to get the credit and I believe it works this way for all modes.
Make sure you start capping when the flag is red or grey as well; depending when you go to the flag, it could have already been captured by your team and the enemy began to push.
Luckily it's just 10 flags for the trophy so hang in there mate
If you select Frontlines from the filter menu, I have heard reports that it will not count towards the trophy. Apparently, it can only be done through "Grand Operations" where there is a stage that has Frontlines.
If you simply play Frontlines, it is the "wrong" set up.
So you have to play hours (if your team sucks) of Grand Operation before this will work. IF it will work...who the hell knows with these bugs.
Not sure what is worse, this or the pathetic War Stories bugs that require to nuke your saves just to finish nearly every challenge.