By the "solenoid not catching" do you mean you hear the starter spinning
with the solenoid not pushing the drive out to engage the flywheel or do you
mean you hear a solid click but nothing else?

In my years in the trade I came across rebuilt starters that many times
would do the latter.  The solenoid on most GM starters does 2 things, it
creates the high current connection to the starter windings and it pushes
the drive out to engage the flywheel.  I've seen many rebuilt starters that
were new or nearly new, including one I put on my own Aurora, misbehave
where the solenoid extends as evidenced by a solid click but fails to make
the current connection to the starter windings.  If the problem is not one
you had with your old starter and if, except for this intermittant problem,
the new starter corrected the condition you were after when you replaced the
starter, in all likelihood you have a defective starter.  However, if this
was the problem you were trying to rectify with the new starter and you're
telling us it's still doing the same thing, then yes, go on to look at the
relay and other associated parts.
Steve G.