If you are using short stacking cables you can possibly try using longer cables.
I experienced a similar issue with EX3400s with a preprovisioned virtual chassis. Even though I believe these chassis may be fairly different with their stacking, if you have cables available it may be worth a shot.
If you care for a full description on my issue:
I would be consoled into the Master and after stacking the switches I would get logged out every 5 minutes or so with the Master alternating between Prsnt and Inactive status, example:
Member ID Status Serial No Model prio Role Mode Mode ID Interface
0 (FPC 0) Inactive NY#########1 ex3400-48p 129 Linecard* N VC
1 (FPC 1) NotPrsnt NY#########2
Member ID Status Serial No Model prio Role Mode Mode ID Interface
0 (FPC 0) Prsnt NY#########1 ex3400-48p 129 Master* N VC
1 (FPC 1) NotPrsnt NY#########2
After many hours (overnight) the VC would stabilize and the VC would work correctly, even after reboots.
I stacked two separate stacks and both exhibited the same behavior, even when swapping out stacking cables. JTAC checked over my configs and did a remote session and said that there was no reason it shouldnt be working. When JTAC tried to replicate my issue they could not, the only difference between my setup and JTACs was that I was using 50CM cables and JTAC was using 3M cables. Stacking cables used in my and JTACs environment were Juniper Branded Molex QSFP+ cables.
After I switched to using 3M cables the VC logged me out once and then has been stable after that. I tested with multiple 50CM and 1M cables of the same brand (Juniper) and they did not work. But, as soon as I used that 3M cable the stack seemed to stabilize.