There are a few things working against Isaiah in regards to getting calls.
1.It used to be that if a player's shot was blocked and there was body (or hand) contact, a foul was called. For the most part that rule either no longer exists or it is not enforced in a consistent manner. Many times defenders either come close to blocking Isaiah's shot or actually block it while making contact. Since refs have a lot of latitude in judgement they can decide whatever they want. This may be a league decision in order to speed up the game.
2. Isaiah often drives through the lane and draws some contact along the way. This too used to be an automatic foul. It is not anymore. Defenders are generally given a lot more leeway in initiating contact when someone is aggressively attacking the basket or moves into their space.
3. Isaiah's height works against him. The refs probably cannot see all the contact that takes place because he is hidden from the action. I reffed one game in my life. I couldn't tell what was going on in the lane with all the bodies mixed together. Maybe if I did it for a living it wouldn't be as difficult as I believe?
4. And finally, even though Isaiah often ends up on the ground after driving to the hoop, refs now take that for flopping or part of his shooting routine even if he ends up plowed into the crowd. Could also be psychological to the refs, making them feel that they are in control by not making too many calls. (just a guess on my part)
Someone like James Harden is going to get a lot more of those driving-the-lane calls because he's taller and harder to miss the contact. And, he's not in the middle of the lane as much as Isaiah. Plus, Harden has longer arms and his herky-jerky moves to the hoop throw defenders off balance. Isaiah depends more on speed and body dynamics to draw contact.
In any case all he can really do is have Danny send videotape to the league if he's really not getting the calls he should be. Complaining to the refs isn't going to help him.