Keep an eye out for red spider mite at this time of year, especially under glass. Depending on local conditions, they may already be well established. This goes for all Viola species/cultivars. Keep it in check now or it can easily get out of control. Chemicals can be efective if started soon enough; natural predators ditto. I find fatty acid plus sulphur the most effective of the chemical controls available to non-commercial growers. Phytoseiulus persimilis is the best predator for UK climates; others are available for warmer climes.

So have a close look at your plants' leaves, especially the undersides.

While you're there, concerning plants in pots...

Are there good roots coming out of the base of the pots? If so, take a hint and consider potting them into larger (or at any rate deeper) pots. Viola sororia and relatives are especially likely to be showing roots now, and maybe since a few weeks ago.