Hey thanks for trying it out and giving some feedback!
I haven't had the "random puzzle" button not work for me in a while, but I've got some good ideas about what the problem might be...troubleshooting time I guess.
Regarding puzzle difficulty and skill level, this is kind of a long answer. TLDR: I'm trying for an elo system that is quite arbitrary at the moment.
My extremely-long-term goal is for this to be like chess puzzles, where you have an Elo rating and are given puzzles equal to that rating. So rating isn't so much beginner -> intermediate -> expert, but rather just a number that goes up or down...maybe not the best way of doing it or just not the best way to present it, but that's what I'm going for at the moment. The player starts arbitrarily at 1000 and should eventually their actual skill level. Of course if you're new to chemistry, this game isn't designed for you (yet, tutorials and lessons will come eventually). There are 2 substantial obstacles with this approach. First, I don't have enough puzzles to cover a wide range of skill levels (IMO, 10k+puzzles are probably needed). Second, chemistry doesn't have Elo like chess does, so my numbers are somewhat arbitrary and based on my opinions at the moment. If you're wondering, I started at a rating of 1000 for the first few mechanisms that are taught in a 2nd year organic chemistry class in university (addition of hydrogen halides to alkenes) and went up from there. Easier puzzles might be simple acid-base reactions.