Banding lines?

[klingsdesigns]

Every once in a while I run into this problem. I really notice it on canvas prints.
On this print attached about every 9 inches this happens.

I dont notice it all time time.

I have had this happen when i pull the material when taping it to my take up real while it is printing. But this is not the case for this. I had enough slack in the back.

 

[klingsdesigns]

sorry the pictures dont really show it like it does in person. Printing on a vp-540.