Actually let me correct myself, it seems that it's fairly common for BSF larvae to pass through frogs' digestive systems completely whole and still alive. I'm surprised, I was always under the impression that they're soft and easily digested. Some keepers have suggested to try poking a hole in the exoskeletons with a needle before feeding, so you can try that. Otherwise I'd say that maybe BSF larvae are more suited to reptiles that chew their food a little before swallowing rather than frogs.