There are several reasons fiberglass resin will not stick to wood. One could be the type of wood. Another could be the proportions of the mix as well as the temperature and timing of the setting process.-
Fiberglass resins will not stick to woods that have been treated. They only stick to untreated, clean, dry wood. Redwood is not only usually treated, but it also contains a waxy substance prohibiting adhesion.
Fiberglass resins, as opposed to plain epoxy resins, have small strands of fiberglass embedded in the resin, hindering the saturation process. For the resin to do its job, it must first penetrate and soak into the wood. The close grain of woods such as oak and cedar prevents that.
As with all resins, the inability to stick to wood can be caused by the wrong proportion of resin to filling and/or hardener.
Improper curing of the resin affects its adhesion to wood. This curing is affected by temperature and timing, both of which need to be controlled.