We have two Workgroup PDM vaults, 2013 and 2014 (we have customers using SW 2013, 2014).

When in Solidworks 2014, I can log into either Workgroup PDM.  In Solidworks 2013 I can only log into the 2013 Workgroup PDM vault. Solidworks 2015 will not log into either vault.

Why there is compatibility in Workgroup PDM from 2014 to 2013, but not 2015 to 2014 or 2013?