Sweet peas need cool soil and cool night air in order to be healthy.
They do like sun, but I find in Texas they are happy with afternoon
shade. I plant my seeds in January.

I also have BOP plants for well over ten years! I douse them with
fertilizer and they've never flowered. Do you have any tips?

I forgot to ask, are yours in pots or in the ground? I think they like big
pots, but like to be rootbound.