Hi there! I’m not from the Prusa Support team but may be able to help.
First of all, I need a little bit more detail. Perhaps a picture? By what you’ve mentioned, I’m assuming that the hotend doesn’t heat up all the way, or at least there is too much resistance, that causes the entire metal nozzle (heatsink, hotend, and all) to shift downward and out of place on your machine.
To combat this, make sure all your screws and 3D-printed parts are secured tightly. Often if the main extruder-body isn’t tightened it’s pretty easy to make something fall out.
Second, make sure that the heatbreak hasn’t broken off the rest of the hotend. Although unlikely (it’s literally metal) it’s possible that mishandling during assembly may have caused it to detach and would explain why the filament is pushing the nozzle down. I’m not sure how experienced in this matter is, so if I sound condescending I don’t mean it. The heatbreak is near the bottom of the hotend, right above the heating block (the block-shaped part lol).
If by what you said above you mean that just the nozzle fell out, then perhaps try heating the hotend up to about 270C and unscrewing the nozzle VERY carefully or tightening it. But, judging by the fact that this is a kit, the nozzle should already be in there pretty good.
The last thing would be an issue with the inside of the extruder/idler. A big giveaway would be the PTFE tube included inside the hotend (or at least it went inside it, not sure if it’s already pre-placed before assembly). Make sure that both the pulleys are basically perfectly aligned with both the PTFE tube and each other. The pulley gear on the extruder door (the one that lifts up) doesn’t matter as it has some leeway but still, it’s better safe than sorry. If the main drive gear is off-centered, it could be pushing the filament at a weird angle, resulting in missing the PTFE hole and instead pushing down on the PTFE tube or grinding.
I hope this helps, and good luck!