It would appear there's leaf rolling insect activity - sometimes its caterpillars waiting to pupate, sticking the leaves together with webbing and grazing on the leaf surface.

5 litre polypockets are not intended for long term use, they're merely a cheaper alternative for some nurseries to use instead of proper pots. I recommend you pot them up properly asap, especially with the approach of winter, if the plants have to stay in pots - be aware their healthy lifespan in pots is probably around five years, even with increasingly larger containers over time. As for the infestation, you'll need a systemic insecticide, something like Provado ultimate bug killer. Before buying, though, inspect inside the leaves to see if you can find anything...