I believe its the cold temperatures we have had this spring that are affecting germination. I've especially noticed, that night-time temperatures are particularly cool. I now tend to start veg seed in pots or containers indoors so I can keep an eye on germination - only then do I plant out. I didn't have to do this when I had an allotment in south east London, but conditions here are a lot harsher.
Of course. some seed has to be directly sown in the beds. And even though I have left things a little late this year, germination has been pretty poor. Some seeds , like parsnip, are not the best of performers, and I often have to reseed these two, or even three, times.