Different types of Comenity credit accounts seem to have different waiting periods between CLIs, or before one is granted. They do seem to be 1, 3, or 4 month intervals, though. If you are denied because you asked too early, the letter will clearly state that you were denied because their "records show that this account has recently received a credit line increase." (My BJ's card is certainly on a 4 month cycle, and I have received that letter a few times, but have received a CLI 5 times from them, every 4 months.)
Has the wording on your denials explicitly stated that you were turned down because the card is too new, and for no other reason(s)? My first denial on the BJ's card said the account was too new, that's it. No "we used credit score A from credit bureau B and you were turned down for reasons C, D, and E" messages. After I received the first CLI, any time I asked prior to 4 months I just received the letter stating that I had recently received a credit line increase, again with no credit score, credit bureau, or any other reasons for denial listed.
If this is the oldest card that you have on two bureaus and it's only 6 months old, that implies that you have a newer card, or cards. Does your credit profile otherwise merit an increased credit line?