Rear derailleur malfunctions Around 1-2 weeks ago I sent my bicycle to my dealer because my cassette wasn't moving properly when I examined it by turning my bike upside down and my shifter shifted 2 times instead of 1 and frequently lost tension while riding (felt like the 1st gear for a few seconds). This happened after I rode on rougher terrain and jumped multiple times on a small ramp made by me and some of my friends (it was around 30cm in height).
I went with my bike at my dealer for a bicycle overhaul. They found out that in the gears there was some thin wire (similar to fishing line) stuck in there. They got my bike fixed and I took it home.
It felt much more smooth and all the problems were fixed. Yesterday I took it for my first ride after the overhaul. I fell down after a failed jump attempt at my local skatepark and I'm pretty sure it didn't land on the derailleur side. Fortunately it worked perfectly just like before and it shifted properly.
Then I took it offroad where I jumped with it and rode it on a small pumptrack. The same problems started again. Shifts 2 times, loses chain tension, etc.
My bike is a GT Avalanche Comp 2019 (2x9 Deore).
How can I fix it?
Glad you are having fun on your bike! Here's a link to adjusting the rear derailleur -- it's worth a look if only to gain some insight into the RD's settings and perhaps what might go wrong. OTOH, you might be able to fix/adjust it yourself :)
https://www.parktool.com/blog/repair-help/rear-derailleur-adjustment
Before working on the RD, it's a good idea to at least brush off any dirt/mud/stuff that might be on the chain and gears. It'll be easier to see what's going on, and the problem might just be something stuck in the gears as with your first issue.