Barry, I know exactly what you are talking about, I have found over the years that there always comes a time when I don't seem to be improving in playing an instrument, My way of getting out of the rut is to change instruments and try the tune on another instrument. In the past I would learn to play a new instrument all together and adapt the styles of the new one to the old one which seemed to give the old one a kick start. (For instance I learned 5 string banjo using finger and thumb picks, I now play guitar with finger and thumb picks and that taught me a completely new style). Another thing you might try is to find a new venue for your playing. I play at music sessions these days and after a few years I find I can play all the tunes that are played at that session. The next thin to do is to find a new session where musicians and tunes are all new to me and I start to "learn" again.
I hope this is of help to you.
Cheers Bill