Monty Don is the person to ask about sweet peas .
Without a photo - I'm not sure .
I've grown huge high hedges of sweet peas for five years with no problem for five years then last year - they failed .
I attributed it to suspect seedlings as I bought them from an allotment sale , a day when I didn't water them as I at the hospital , the wrong place - my heathers pelargoniums love it but it is possibly too dry /hot , insects or a virus !
There are about 50 worldwide viruses which affect sweet peas, eleven in the UK of which four are prevalent .
Normally, I'd investigate further but I decided to destroy them .
You could either using sterilised secateurs (before and afterwards) give them a trim ,some more Tomatorite and perhaps check for aphids which you can easily treat with washing up liquid diluted in water and sprayed on or remove them .
It sounds like mosaic virus and can't be treated .
Disappointing, I know .
I'm sure that a sweet pea expert knows more than I do .
Find someone at your local gardening club who grows show quality ones . They know every trick in the trade .
Some plants just don't do well every year .
But are wonderful- the next .
Good luck !