Please paste "show chassis hardware" and "show chassis pic fpc-slot x pic-slot x" so that we know the exact module we are discussing.
A few possibilities:
1. Transient HW issue. Something went wrong during FPC reboot process or insertion of SFP. This is most likely your case, since a reboot can resolve the issue
2. sfp sfp+ incombatibility
3. There're corner cases that a particular Juniper SFP is supported for platform 1, for example MX, but not supported for platform 2, for example EX. Those are really corner cases and we have been working on to close such gap
If this is constantly repeatable, I'd suggest to open a JTAC case to track such issue