They probably gave you the correct advice, but there's too much information missing in your posts to be sure. The 7777 is very susceptible to internal overloading in the presence of even moderate signals, causing a complete loss of reception. It's usage should be reserved only for those who have the test equipment and knowledge of when and how to use it, IMHO. FWIW, CM recommends it ONLY for locations where ALL stations are beyond 80 miles....
The RCA's power inserter is not capable of supplying adequate operating power to a 7777 so it's pretty likely that the 7777 is operating in a power-starved, lower gain mode than it was designed to function. Ironically, this allows the amp to function somewhat but at the expense of a severely taxed power supply which is apt to fail in relatively short order from overheating.
Last edited by ADTech; 15-Jun-2016 at 8:03 PM.