This KB is a troubleshooting guide for this scenario.

From Mist portal you can see that the upgrade starts, the % bar fills but the upgrade seems to not happen, no specific error seen.

To understand what could be happening at the SRX there are the following CLI commands:

show system snapshot media internal

show log install

request system software status

For example with these you could identify issue of KB79077 [juniper.net], or also if the upgrade failed due to a storage issue, or if it failed due to a configuration compatibility issue, or any other upgrade exceptions.