Hey there,
I just experienced similar problems. There are a few things I would think about if I were you...
Has there been any change in the bottling process, racking beer on top of priming solution and not mixing gently with sanitized spoon, rather than racking first and pouring in the solution then mixing gently. Have you cold crashed a little to long so there are not as many yeast in suspension? Are you using too much sanitizer that it kills some of your yeast as you bottle?
My issue was the uneven priming sugar distribution due to not mixing in the sugar solution as I normally do...
-Josh