Symmons Shower valve alwys hard to turn.

[homeowner Tom]

The white diverter valve on my Symmons Tempertrol diverter is always hard to turn. It began two years after it was installed, The replacements have the same problem. Is the valve not inserted far enough? I installed a my third one with silicon grease it still hard to turn. I don't like forcing it because it breaks the more expensive silver handle, that has happened twice This tub shower is infrequently used Any suggestions?