Sweet pepper disappointment

For years I've been successfully growing good crops of tomatoes and cues in unheated greenhouse. Every year I also attempt sweet peppers (various varieties) but they lack vigour and number of fruit is low (like 2 or 3).
I treat them the same as toms and cues -soil, watering etc- but something isn't right. What extras do peppers need?
I'm new here on forum, so any suggestions very welcome. Thanks

I grow peppers from seed I originally saved from a Trashco's Ramiro pepper  (but you can buy them on ebay)
They're easy to grow - I start them off on my window ledge (so they get heat from the radiator underneath) and keep them indoors until the greenhouse is warm enough. They then get potted on (the size pot you'd grow a tomato in) but they do need feeding weekly, and careful watering, for them to produce more than 1 or 2 fruits.

Peppers are fussy about temperature in a way toms and cues just aren't. They basically sulk below about 15C so in an unheated greenhouse they're always going to be behind. Starting them really early, like January or February, on a heat mat and keeping them inside until the greenhouse is properly warm makes a massive difference.

Peppers are of the ilk to be permanently stunted by low temps, you've gotta be careful when you plant out basically.

Tomatoes and cucumbers don't mind so much (cucumbers will sulk but usually spring back if they survive).