Inaccessible Pex Leak in Radiant

When I first installed my radiant heat (over wood sub-floor, 2x10's, and insulation, which is over a former patio, and under wood sub-floor and hardwood) I tested for leaks. There was 1 "nick" from transport of the 7/8" pex, which was repaired with brass barbed couplings and pipe clamps, and then the system was tight. Since covering it and using it for 4 or 5 seasons, it has developed a small (based on the small pressure drop when I've tested it) leak, possibly another nick, or leak at the repair site. I've tried "Silver King", a boiler seal product which the manufacturer said would work, but it hasn't. Does anyone have any experience with the need to seal Pex from the inside? Thanks.