GRP is strange stuff. I used a plug to make a female mould last year. a few days later I waxed it well several times and made a part in it - a couple of days later there was no way I could get it out and both the part and mould were scrap.
Made a new female mould off same plug, and this time I let it cure for 2 weeks. Similar prep with same wax, and when I made the first part it almost fell straight out.
It seems (reading on the 'net) that if (in this case the female mould) isn't
fully cured then some cross-linking of molecules will still take place even with good release prep. If the weather's sunny I leave GRP (if in a good, stable mould that won't move) out in direct sunlight to cook it nicely