Two different areas of the garden is probably the answer - one might get more sun, (sweet peas hate a lot of shade) or one might get full sun (and that's been a bit strong in the last month or so), or they're different varieties?

Thank you for the response! They are both in full sun and all the same varieties I just split them into the 2 seperate areas. I have removed some of the heads of the flowers that have died, which I thought you were supposed to do? Or am I wrong?

Mine are also taking a long time to flower - but we have had such strong winds here just lately and I wondered if that could be the problem - blowing buds off before they developed properly ....

Deadheading is critical - those flowers turn to pods in a heartbeat, and once the seeds form, the plant doesn't bother to flower again, so yes, deadheading essential. Something is clearly different in the growing conditions between the two lots of sweetpeas, up to you to work out what it might be from several (or a combination of) things such as dryness, level of nutrients in the soil, levels of humus in the soil, wind, shade (are one lot near a larger shrub or tree that creates a bit more shade, for instance), etc. always assuming they're in the ground and you haven't got one lot in a pot.

Thank you! Yes this is what I am struggling to work out..No it is not in a pot.