Welcome to the forum.
Take it it's hypalon?
Ussuming you're measuring the glue and the activator out precisely at 25:1, then the only other contributing factor, and it's a major one, is ambient temperature and humidity.
You want good ventilation and low humidity (otherwise the glue blooms) and obviously out of direct sunlight.
Brush wise, trim down a brush so it's easier to work with. With contact adhesive it's about achieving two thin coats on the patch and boat, degreased first with Toluene solvent, etc.