It sounds to me like one of two things. A. it has not had long enough at a high enough temperature to cure yet. or B. you did not clean the hood well enough before you sprayed it.
Naval jelly is nasty stuff, it works well, yes, but, it is a real pain to get it all off the panel. And, MP172 has a 1 week window at 75 degrees. But, because you say it is not sticking when you put filler on it tell's me, You didn't get it clean. I'd say, sand it all back off, clean very thoroughly with water (yes, water) then wax and grease remover, then try again. Remember that you can wipe your filler right over it (within a week) without sanding. And I've used MP172 several times and it works.