There are so many variables here: temperature, humidity, how thick a coat of stripper was applied and with what tool. Most strippers, Interstrip included, are designed to be applied in a HEAVY coat so that the outer layer skins over thereby tending to seal in the solvents in the inner layer. If the product dried too quickly (too hot and/or dry or too much wind), wasn't applied thickly enough (after the outer layer skinned there wasn't much left underneath so that it, too, all evaporated) then the stripper would not be effective. Still, it should have had at least some impact on the paint in some areas - those where it was thicker than others. Of course, we also do not know exactly what that paint is. Coatings such as Awlgrip and other two-part epoxies and modified epoxies are TOUGH (which is why people are willing to pay so much for them) and you may be up against something like that. I'd suggest you try again, this time in shaded areas with as thick a coat of stripper as you can get to stay in place on the hull. Then test it from time to time to see if it is working. Try a small area first so as not to waste too much stripper 'till you get a feel for how to work with it.
So far as it adversely affecting the laminate underneath, since you are working on an old boat, proceed with caution. Some early resins may be damaged by the extremely "hot" solvents in strippers whereas the newer epoxies are harder amd more dense.
As others have suggested, that boat will become a part of you (and you it). Great relationship to have!
I know - I love my daughter - ....errrrr, BOAT, dearly!!
Have fun!!
BTW - some of the new soy-based strippers are very effective (Cap't John's Soy Strip comes to mind) and are environmentally friendlier than others.