I used to start my cukes early but I've stopped doing that. One year my husband planted the early start ones in their own trench. in another trench beside them he planted seeds. In 3 weeks the seed started ones had caught up to the early started ones. They both flowered and set fruit at the same time.

When I thought about it it made sense to me. When you transplant it takes a plant a week or 2 for the plant to settle in get used to the light and soil and start growing. The seeds germinate and just keep on growing. Now we just plant the seeds and it gives me more room under the lights and in the greenhouse not to mention the cost of seed starting mix. So don't despair if you don't have success with your plants. You can always plant seeds

As for your cukes I would say the temperature is too cool. Tomatoes while liking a warmer temperature will tolerate a cooler temperature. Cucumbers however like warm soil. If you have a heating mat put it under them or try to raise the temperature somehow.

Cukes (as well as other members of the family) are normally direct seeded and as mentioned don't always tolerate transplanting well. Plus they are very soil temp sensitive, fast growing, require frequent transplanting to larger containers because of their fast growth, and are easily stressed and stunted, yet more reasons for direct seeding.

Not everything responds well to indoor germination and some vegetable varieties won't tolerate it at all so it pays to know what varieties should be direct seeded for best results.

Dave