A question about how my strings sound.

Hi guys,in the last month or so,my 1st and 2nd strings(E and C)have started to sound odd when I'm up around the 12th fret marker and slightly higher.
The only way I can think of to describe it is they sound like a badly tuned pub piano or a honky tonk kind of sound if that makes sense?They're not ringing clear as they should.
My digital chromatic tuner is spot on..not sharp or flat..when I check the tuning,but I only do that on open strings.Any ideas please re what's causing this?
Thanks in advance
Roy

[Danny Mitchell]

Hi Roy,
I wonder if you're not damping the strings adequately behind the bar. Up at the 12th fret this would cause a chorusey, detuned effect.

Hi Danny
Looks like that's what was missing,I wasn't damping them enough behind the bar.
I've tried it and now it all sounds better on those strings.Cheers for that!
Roy