Earthakitten - your cucumbers may be suffering from lack of nutrition in the soil if you're keeping them well watered. Try a liquid fertiliser like Tomorite. I'd also just water them well once a day but spray the leaves as cucumbers like a humid atmosphere to thrive.
Spring onions can sometimes be slow to germinate and I suspect that this current heatwave is making the soil just to hot for them to germinate, even if you're keeping it moist. I'm having problems with several of my successions sowings at the moment - carrots, beans and lettuce so it's probably the high temperature and nothing to do with your ability as a gardener.