Why do the pieces keep getting stuck behind eachother?

it was good but...

I thought it was good but just 1 puzzle? Maybe try a few more

Quite fun if....

Quite fun if you don't know what to do with a minute of spare time... transparent pieces were a good idea, it made the game less messy. But it was short , not a challenge for even my guinea pig, the music stopped after i matched first piece, so sorry but i can;t give more than 2/10

A good first step.

You admit this is your first game (and it shows), but it's a good first effort. The game works and has a win condition. In no particular order:
You may wish to have a smaller puzzle area, and an outside area where pieces can be arranged or assembled. This prevents pieces from bbeing locked in place and potentially blocking access to other pieces.
Transparent puzzle pieces don't seem like the way to go. A bit visually confusing. ( Also see above. )
The choice of background music was good for a title screen or victory, but a more subdued/ambient music selection the game would be nice. Something that repeats to fill in the empty space while working on the puzzle.
I'm not sure the game needs a high score table. You probably want to focus more on developing the game than incorporating that, particularly because there is such a noticeable gap in the UIs.