What is Download mode?

I have an AllenBradley CompactLogix v20.12PLC that keeps going into
‘Connecting Download Mode’.
This will last for days and days and then correct itself.
While in ‘Connecting Download Mode’ the tags are not updating.
What is causing this and how do I correct it?
I have 3 other CompactLogix PLC on the same server/gateway and they operate issue free.

From what I can see the legacy CLX driver reads the processor mode as part of its periodic status request.

If it’s not RUN or PROGRAM then the connection state is considered invalid. One of things the mode can apparently indicate is a download is in progress, but I’m not sure where this is documented, or how these modes were determined.

Are the other 3 PLCs the same version?