Adding insert plug-ins only works intermittently

[cytokinesis]

New to Pro Tools and love it, but 90% of the time it won't let me add plug-ins to a track. It lists them all just fine, but when I select one to assign, it doesn't do anything. It doesn't crash, doesn't slow down the processing, it just does nothing but leave me frustrated. It also doesn't seem to matter what plug-in or plug-in type it is, not even which insert channel I'm selecting.

The other 10% of the time it adds the inserts just fine, and they work perfectly, but even in the same session sometimes it won't let me add any more and I can't for the life of me find any patterns with when it will and will not let me add inserts.

I can't find this in the Avid Troubleshooting documentation and Google hasn't returned anything useful with the searches I've done, so here I am. Is anyone else having this issue? Does anyone know of a possible cause and/or solution?