If you've got Frontier miles that aren't posting, you should take that issue up with Frontier, not CO. I have learned over these many years that when partner miles don't post, I call the PARTNER. Saves time and wasted words.
I had a problem with THRIFTY miles not posting to my FF account. I now carry pre-addressed envelopes with Thrifty's address on them in my briefcase. I complete my car rental, make a photo-copy of the rental agreement (at Thrifty's counter), add my FF account number to it, stick it in the envelope, and send it in. I have probably done 20 or 30 of these now and been 100% successful (knock on wood!).