new NVMe SSD slips out of the mounting slot

If your NVMe SSD is slipping out of its mounting slot in your Dell laptop, the most likely cause is a missing or improperly installed standoff or screw, which are crucial for securing the SSD. 
Here's a breakdown of the problem and potential solutions:
1. Check for missing or incorrect standoff
NVMe SSDs are secured by a standoff (a small, typically black or metal post) and a screw that fits into the standoff, holding the end of the SSD down.
Dell laptops often use different standoff heights depending on the motherboard and M.2 slot configuration.
If the standoff is missing or the wrong height, the SSD won't be held down properly and can slip out. 
2. Ensure the SSD is properly seated
Even with the correct standoff and screw, the NVMe SSD needs to be fully inserted into the M.2 slot at an angle before pushing it down and securing it.
Make sure the notches on the SSD align with the tabs in the slot before pushing it in firmly. 
3. Consider other potential factors
Environmental factors: Excessive vibration or physical shock can potentially dislodge a poorly secured SSD. Ensure your laptop is used in a stable environment.
Thermal throttling or overheating: While unlikely to cause the SSD to physically slip out, overheating can lead to performance issues or drive disconnections, which might be mistaken for a physical slipping issue. 
Recommendations
Power down and disconnect: Completely power off your laptop and disconnect the power adapter and battery before attempting any internal repairs.
Open the base cover: Carefully remove the base cover to access the internal components, including the NVMe SSD slot.
Inspect the mounting area: Examine the area where the NVMe SSD is supposed to be mounted. Look for a small screw or bracket that secures the end of the SSD. You may be missing a standoff or screw.
Re-seat the SSD: Carefully align the SSD with the slot, insert it at an angle, push it down firmly, and then secure it with the screw and standoff (if present).

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Jesse,  All excellent appropriate suggestions!!   I tried all of them yesterday, three times.  But it still pops out.  I notice the motherboard flexing a little bit to the right of the SSD.  It may be the slot is damaged.  I was able to fit a different SSDs in to test the fit and both were snug in the slot and locked down with the set screw.  I didn't try to boot from the second SSD because I use it in my desktop and it has a small heat sink on it, so I wasn't sure it would allow the cover to close properly.  The set screw appears to be the same one I see in the Service Manual, I believe.  So I have to assume it is a damaged slot.  I considered fashioning a small washer from the plastic non-conductive plastic the SSD was shipped in to hold it in place, but not sure that would help and don't want to impair the SSD.   So I found an alternative solution - I installed Linux on an external SATA 2.5" SSD and modified the BIOS to recognize it.   This worked nicely.  Not as fast as an NVMe SSD but better than running off the HD.  Gives me the option of running Linux on any Intel machine once I add the device to the BIOS.    

Thank you for the prompt, detailed and thorough troubleshooting tips!!!