Felpros are also fine. Like Marky said retorque and sometimes you gotta check them and retorque is necessary.
If they were fine for a while and then you developed a leak then you gotta check them every once and a while.
If not then maybe the flange area is not flat.
If the bolts keep getting undone then you have these header bolt gnomes that come at night and loosen the bolts to mess with you.
Also they have locking bolts like Stage 8 bolts.
1989 Sonic Blue Mustang LX Coupe, 349 ci, TW heads, TFS Track Heat intake, 75mm Accufab TB, 80mm ProM, 30 lb inj., Jay Allen cam, TKO, 3.73, 31 spline w/ TruTrac, MAC 1-5/8" LT headers w/ h-pipe, 2-1/2 flowmaster, electric fan oh and a Moates QH
392 rwhp 424 rwtq....Mustang Dyno
"There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves." -Will Rogers-