Hi Solara

I happened to me recently too. Every belts were new and after 15/20 runs, the belts start to relax.
You can tight the side and rear belt a bit more but keep them free enough not to get the transmission unsmooth.

I personaly bet it comes from the front one like on my own car.
To check if i was right, I prevented the rear wheels to spin on the board of a table and I turned gently but fermly one front wheel (with spool only). And bingo, the front belt is now loose enough to skip.

You may use this link as a permanent solution !

Enjoy
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