Probably a bad mixing. However, it is not a good idea to use 5 minute epoxy where high strength is required (like on a firewall) as it is to easy to do it wrong; a bad mix, a proper mix starting to cure befor application is complete etc. Also, the slower epoxy are quite a bit stronger for two or more reasons; two of them being that the slower epoxies penetrate more and the resulting molecular chain of the cured epoxy is longer thus stronger.