i hate to chime in and here ..... BUT ... i am gonna bet that the epoxy is still soft inside your stock.... and will no doubt continue to soften. I could be wrong... but i do what yer doin every day

Bondo fiberglass products are polyester based..... PC7 is epoxy. A barrier coat MUST be applied over epoxy before a polyester product can be applied to it.... or the polyester/epoxy blending causes a chemical reaction that destroys the epoxy
for guys wanting to play with thier fiberglass stocks..... go to a plastics or marine shop. PC7 is a good product, as is marine fix fast from goop. System 3 eopxy resins can be purchased in small repair size.... perfect for this application, from most fiberglass/marine/plastics retailers. Use the same cloths as you used, or kevlar sheeting if you can get it. Wait 2 or 3 days prior to sanding for a full cure.
I don't play a custom stock maker on T.V. , I am one in real life hehehe