String breaking. I need suggestions.

[black velvet]

I am having problems with strings breaking at  the nocking point.
 I am using F/F 14 strands, flemish twist. 54# bow. Serving is not wearing, the sting is breaking under the serving without warning. Once while the bow was set aside. Twice upon release.
Any help would be greatly appreciated.

[black velvet]

Forgot to add I am using tie on nock ppoints.