When this "all of a sudden" occurred, had the temperature dropped enough to require a coat when shooting?
If so, it's very likely too much grease in the striker spring. Remove the striker assembly from the bolt assembly like you would do to change the bolt head. Spray the striker spring and assembly really well with degreaser. Let it dry and relube with a light oil. Also spray out the inside of the bolt body. Reassemble and give it a try.
To check if the striker is sluggish when cold, place the bolt assembly in the freezer for an hour. Pull it out and release the striker with the safety when the striker is rotated into the firing position (striker can move forward into cocking notch in the bolt body). If it is at all sluggish or not as sharp as when warm, the grease around the striker spring is slowing it down.
The firing pin in the bolt head should move freely also. It should not have any of this thick grease in it, just a very light oil as lube.
If degreasing it does not make the problem go away, contact FN for service.
Hope this helps.
AG