Sowing Veg Seeds.....What Am I Doing Wrong?

Here's a thread with some pictures of my Leeks after they came out of the fish box
http://gardenerscorner.co.uk/forum/threads/leeks-musselburgh.50333/page-3#post-684016

 

Nearly a year on and I'm still not having much luck.

I've down on my little veg patch in my garden this year spring onions and some carrots.

Eventually a few seedlings popped up and I began putting slug pellets down every week. Yet still my carrot seedlings have all disappeared (I suspect slugs) and my spring onions have gone brown at the tips. I'm having a bit more success in containers but things are still not as good as I hoped.

The trouble is the huge amount of seeds I am sowing only an eighth are making it through and then these are eaten even though I have tried everything under the sun to give them a good start.

What else could I be doing wrong?