Right. I don't think IxFPs have difficulty understanding things... but ExFPs do.
I dont think ENFP's really have a problem understanding things. From my experience, they seem to be extremely smart(although they may come off differently) and good at just about everything.

Now, ESFJ's and ESFP's, on the other hand, they just DONT GET IT. ESPECIALLY ESFJ's. You can say something so clear and to their face and they'll still respond with "huh?" or "I don't get it." It's funny how see through and open-book like they are with their facial expressions. You can see the bewilderment on their face. ESFP's aren't quite so bad... and I like them, so no offence in their case. They don't have an insanely strong inclanaion towards constant lying/denial patterns (that they think they pull off so well), that they somehow justify within their own mind, like ESFJ's.