Sweet pea germination rates

I've been sowing lots of sweet pea seeds but not getting very good germination rates - less than half, I'd say. I have soaked them in warm water for about a day immediately before sowing, and sowed into seed compost in cells, kept reasonably warm and covered (usually in a plastic bag).
Is this normal or am I doing something wrong?

 

I don't pre soak mine. I pop them into loo roll tubes which are stood in a tray which is always dampened and that's in a cold G-House,until this year they were raised in a cold frame.I use a 50/50 mix of sharp sand and muti purpose compost and so far have had 100% success. One year I grew some in the house but they were leggy and rotted off once outdoors.

 

Looks like they are maybe too warm as I've been starting them indoors, and perhaps the addition of some sharp sand would help - thanks very much for the advice Paladin