When the weather warms up, they will take off.
Linda C

Don't give up hope yet.
One thing to remember about sweet pea growing in temperate zones, is that these may be sown in the Fall, which gives more time for root development so that when the weather does warm up, they are ready to jump up.
I do sow in the Fall, but this Spring I also purchased one sweet pea plant in a 4 inch pot to include among my others mainly to get that certain flower color. The new addition is indeed lagging behind the others. But it is perking up.
I am getting flowers on the Fall sown plants now. I expect in about 2 weeks the additional plant will start to flower too.
Some bloom fertilizer might help your plants. Or earthworm castings mixed in the soil is a good stimulant too.

I hardly think you could autumn sow in PA; given their hard snowfalls or is this in your area? Autumn sowing is for those who have mild winters.

You can plant out when danger of severe weather is over. This does not mean no frost; actually sweet peas once established can take a frost or two. It does mean severe prolonged freezes, however. They take awhile to establish themselves so give them a good two weeks to a month; don't fertilise until after the first flowering. When your summers warm up and the weather gets a bit more constant, you will have flowers. Make sure you keep them moist and their heads in the sun for at least six hours a day; preferably more. Keep them picked once they start to flower; or they will go to seed thinking it is time to sleep and you will not get any more blooms.

Thanks, everyone... this was a new experiment, so I guess I will just have to hope that they grow a bit more! I am actually in SW PA near Pittsburgh, and we have fairly hard winters (fairly cold, but not a ton of snow). We are just off a river, too, so the cold winds do a number on my outlying garden! I haven't had too much luck with fall sowing, though some sunflowers came up this spring. I can certainly *try* it, though!

Part of the problem here was a really late snow and ice patch right around Easter- a lot of stuff froze off and is doing poorly. I planted these well after that, but they just don't seem to be doing well. They look healthy enough, green and such, but just aren't growing. I had hoped to train them over a larger trellis, but at this rate they may not even get past the one footer I have them on now. The weather has been pretty sunny and warm (70s and 80s), and I have watered just about daily when we have a stretch without rain. They get full sun for most of the day...

Thanks for the advice, I will keep an eye on them!

Now that your weather is becoming more even and warming up, they will take off. They often are slow to start, and will seem to sit there. They are developing roots, and most people plant in autumn when they can so that they will take the time to develop strong ones. Since you cannot, you planted when it was safe to; so you have to wait for the same development. Next year, you could start them indoors and then put them in a cold frame over winter or in January; where they will be grown on cool. Then put them in the ground and you will have superb peas earlier!