A bit early maybe? This may nelp http://www.bbc.co.uk/gardening/basics/techniques/growfruitandveg_growingsweetcorn1.shtml
Jay, cukes and corn need the most warmth of just about all veg to germinate. It's the soil temp that's key. Where are the cuke and corn seeds sown? If in cells or pots, try putting them into plastic bags to generate a mini hothouse.
Sweetcorn are so easy to germinate. Wet/dampen some paper towel and place in plastic box. Place seeds on paper towel. Place top or clingfilm on box. Leave box at room temperature. Examine seeds every day and as soon as you see the root emerge lift the seed very carefully and place in deep module or pot of compost and cover with loose compost.
Job done. The seeds will start to germinate in a couple of days.
From some seed suppliers you will get 100 percent germination; Seeds of Italy being one of them. From others you will have a very poor rate of germination. The fault is with the seed/seed company not the method.
Do not start sweetcorn off too early, it should not go out before all danger of frost is gone.
My sweetcorn has just germinated. Started the seeds in toilet roll tubes full of damp mix. Our daytime temps here have hit the mid-20sC. I left the rolls in a crate wrapped in bubble wrap for extra warmth on the terrace during the day, put them under cover at night. They all germinated within a week.