Cheers! Well in my case, I can just change the wording on the clue cards. Give more or less of a hint based on the words or pictures I use.
Of course, there are certain puzzles in the desert chapter that need solving without a clue card, and those were always the toughest to modulate. I found exactly the same... my testers would either get it immediately or get stuck for 20 minutes. It's not the end of the world if one puzzle presents more of a challenge to a certain player than the others, as long as most everyone is getting through, and its not consistently ONE part everyone gets stuck at. I just have to trust that by then they are engaged enough to keep thinking about it for 20 minutes.
One more advice: don't trust your own feelings on how hard the puzzles are. There is no substitute for playtesting.
I will follow your game! I want to play a puzzle game like mine, that's why I made it, haha.