Hi John, it seems that you have also discovered one of the quirks of the genus Viola (among others). What you are seeing are cleistogamous flowers- blooms that never really open, but are capable of self pollination. I had several seedlings of Viola variegata that spent time in the unheated greenhouse in midsummer. They grew well, and sent out flower stalks which never bloomed, but set seed. The greenhouse was probably too hot and sunny for their liking.
Apparently several species of Viola may produce these self pollinating 'blooms' when conditions aren't exactly ideal- to insure seed set. Perhaps exposing the plants to cooler, shadier or more humid conditions might induce traditional flowering.
