Why aren't Croatian "plam" (meaning "flame") and English "flame" considered to be related, or at least possibly related? They mean exactly the same, and they seem to fit phonologically. Croatian 'p' regularly corresponds to English 'f' by the Grimm's Law. So, why is the English word being a borrowing from Latin "flamma" and Croatian word being related to "pepeo" (ash) considered a more likely explanation?
English flame isn't a native English word, but a loan from Old French flame; Grimm's law doesn't apply, as it had long ceased to operate at that point. The initial f actually reflects PIE *bʰ, which would yield b in both Slavic and Germanic but became f in Latin.
Both English and Serbo-Croatian do have native descendants of the PIE root *bʰel- that yielded flame: English bale[fire] (now archaic), Croatian bijȇl 'white'. The similarity between plȃm and flame, however, is just a coincidence.