With my limited experience, sounds like not enough CO2 possibly. Although Vals are pretty easy to grow anywhere. Would help to know more about your tank. I recently had a CO2 issue that had been going on for close to a month before I figured out what was going on. I put some Rotala Indica in my tank and the stems never straightened out from the way they droop a little when you first put them in there. Fixed my CO2 issue and within a few days those stems were as straight as a nail. The CO2 issue didn't affect the other plants I had in my tank, but I did get a bad bout of BBA.