Okay, I missed this when you replied...sorry. I wanted to make sure you had no walnut or butternut trees in the garden area. Walnuts will kill everything in its root zone. They make a chemical that suppresses the competition for nutrients.
Is it warm enough? cukes like it pretty warm, what is the soil temperature, over 60 ( you need a soil thermometer to check, they are about 6" long) and the day time temps over 70 with warm nights? are they in containers or in the ground? In the ground the soil takes much longer to warm up. in the garden it could be the temperature.
Are you direct seeding or starting plants to set out? they do not like to have their roots disturbed, either.
How big are the plants you are setting out? I don't put out plants that are any bigger than the one leaf stage. No tendrils no vining. I find they do better putting them out at that stage, carefully remove them from the cell pack or plant them in peat cups and make sure you bury or tear off anything that would be above the new soil line. The exposed part of the peat cups will act a s a wick and dehydrate the little soil mass. making them struggle for the duration of life.
maybe there is some info in this page from OSU that will help, too.
http://ohioline.osu.edu/hyg-fact/1000/1608.html
Don't worry about the stuff from the nursery/garden center. You have no idea what was sprayed on it or used for soil or fertilizer, maybe not even the same variety. I grow lots of stuff and I grow as conventionally as possible. I don't use any growth control agents in my greenhouse. What you see is what you are taking home today and the same for the rest of the summer. A lot of greenhouses use a growth regulator to keep the growth compact on many plants, not so much vegetables, but none of use has any idea what they did to produce those cukes so we can't compare or tell you how/what they did it.
I planted my hightunnel cukes about 4 weeks ago and they are still struggling even though it has been really nice here. hardly past the one leaf stage on a few and a few aren't more than 3 leaves yet. I think it is really not warm enough at nighttime for them.