Hello zakzak. There are several reasons why perennial sweet peas don't flower. They could be suffering from shade - they love being in sunlight; excessive heat which we have had a great deal of this Summer and connected to that, being too dry.
Do you have your plant in the ground or in a pot?
Cacoethes: An irresistible urge to do something inadvisable
Ok thanku all my plants are in pots as I can't dig and do normal gardening. I've just looked at it and it looks like there's one flower going to burst open in next couple of days.
You might have better luck if you restrict the size of the plant and not let it sprawl. I have several planted out in the garden and I try to keep them to head height. As Hosta says, water and feed lots. You might be better off with Annual sweet peas next year. They are hungry and thirsty too.
Which part of the UK do you live zakzak? You can see where I live and 2 of mine have just started to flower. Buds are only just forming on the other two. All four of them are looking very healthy.
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They also have pretty impressive root systems so pots are not ideal.