As is explained in this thread: http://community.logos.com/forums/p/69149/502211.aspx#502211 when I update my reading plan on IOS it does not cut across to my 5.1 installation, it does however update the plan on Biblia which shows that it is not an issue with IOS.
If you update one entry manually on 5.1 it suddenly fully updates but once gain does not sync with further updates in real time.
In the other thread I was told that this was fixed in the beta and would therefore be fixed in 5.1.
What is strange is that I have a beta instillation and in RC3 (and before) the sync issue is indeed fixed, but now that 5.1 has gone live my non beta instillations (on two separate machines) still do not sync reading plans.