I don't think it's the pots, nasturtiums don't like to be transplanted. It's best to sow the seed directly. I've had a lot of success growing nasturtiums in pots but I've never had success buying a start from a nursery and transplanting it. Nasturtiums are a good flower for collecting the seeds from your own flowers and using them the next season, I've been doing that for years.

The potting soil probably had too much fertilizer in it. Nasturtiums bloom best when they don't get a lot of fert. They will bloom plenty in pots. I have grown them that way.