I had a similar situation - which I managed to get working again. Short version - break BT, Tread and Wireless conenctions except on one device.
(Its not a good solution for me - but it is where I am at preset - I want to use my ipad for the Tread map - but need to use my phone for synching. I'll get there. Eventually)
My thinking behind this.
I tried synching with my ipad as well as my phone. It didn't go well. It sort of worked, but not completely, and I couldn't work out what it was doing. Some data appeared, some didn't.
If I forgot the BT connection on one device AND disabled the Tread App on the same device, then it worked.
I think, and it is only a thought - but I think it may have some substance - that the Tread app on the device (phone/ipad) is able to communicate with the Main Explore Server over wifi. The Explore database and the XT2 must have some way of indicating that the data has been transferred - otherwise when the XT2 gets a new track, what is to stop it from sending it back? And vice versa.
So if the Explore database has a 'flag' against a track that it has been sent out to the iPad Tread app - then it doesn't get sent again. And my XT2 didn't receive it.
Try deleting the Tread App completely from one device. Also break the BT connection with the XT2 to that device And the BT connection on the XT to that device. ie forget the connections.
The on the other device - redo the BT connection and set up the Tread App to talk with the XT2.
I think that this may solve your problem.
I have yet to work this out properly, 'cos I use my phone with the XT2 , but the route planning is better on the ipad. I need a more robust way of dealing with it.
The only other thought is to check out all headings in collections - including 'recently transferred' and 'unorganised'