incr rman agent oracle backup fails but full completes successfully

I have an incremental oracle backup running on red hat linux 2.6.18 (client sftw vers 7.5.0.7 ) and the master ( no media server in between) is running solaris 5.1.0 ( sftw vers  7.5.0.7).  The stream fails with status 13 and the parent job fails with a 6.  The full runs successfully every time, and they are pretty much the same scripts.  Any suggestions would be appreciated.  Will add logs to post.