I have had a repair a number of Mk5 safeties over the years. In most cases the safety was not stoned to proper specs when it left the factory.
On a MK5 the safety should cam the striker back about .010" when it's engaged. If the safety only blocks the striker without moving it back it works until it wears just a bit. Then the sear nose is allowed to get in front of the striker and will not allow the safety lever to be set into the "on-safe" position.
Worse is when they get to a halfway point in wear, and the safety then can allow the rifle to fire when you flip it to the off position.
The repair is not hard to do, but you must know how to do it right or you can create a dangerous situation.
It involves stoning either the striker or the sear nose (depending on which one needs to be repaired) so that the safety cam can grab the striker and pull it back as it is locked. The thickness of a playing card is fine, but it MUST be allowed to cam the striker back and lock it.
If you do not understand the internals of the MK perfectly I would advise not trying it yourself. Besides, on safety issues I'd bet Weatherby would be delighted to fix it for you, and I'd bet they'll do it free.