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?

No, sorry, I don't. Mine aren't old enough to bloom yet. I did see a
potful blooming madly at the nursery where I got mine, though. I've never
seen so many blooms at once on a BOP! The plant had been abandoned in a
mostly sun/partly dappled shade area, grown through, over and out of the pot
into the ground. The plastic pot was still surrounding it, but was cracking
from root pressure. I don't know how long it was there.