Did you nip out the growing tips too soon?

The lady above is asking the same question

Lots of factors. Depends on the variety and how you are growing them, and when you started them off.
Sweet peas reduce in length as the season progresses.
To have the best chance of keeping long stems you have to give plenty of water, especially so this year with the heat we have been having. A good soaking early morning is the best time.Even the top growers are noticing a more rapid decline in stem length this year, and reduction in blooms per spike to 3 instead of 4.
A good dose of low nitrogen soluble fertiliser( Chempak, Elixir, Garden direct all supply them) about a month after they have started flowering can help. However, others never use fertiliser and rely on soil preparation. It depends on your soil.
So in a nutshell, it's trying to prevent reducing stem length before it happens by doing the above. Once it starts, they won't pick up too much.

but some of mine then produce longer flowered ones as the season develops. I suspect its the plants response to get a flower out and ready to set seed.

Stem length can be aided by layering the sweet peas. Less gravity to get water to the growing point, so they can pick up a little.
Certainly won't go from stalks 6" long back up to a reasonable 12"-15" for showing.

Thanks to all re sweet pea length I am going to really soak them and see what happens. I would mention that I grew these from seeds planted in October in the long thin tubes and the difference from bought plants has been vast....really thick stems and hundreds of blooms but as I have said now getting short....hey ho