EFL writing (children)

This time I really need your help :-(

Here's the thing:
Among others, I teach 10-year-old children. They have been studing English for two years (spoken), and now it is the time to start writing. Since September I've put a lot of effort to teach them write, but the result is very poor. The problem is that the Serbian language has one-letter-one-sound correspondence. Obviously, I don't expect them to write every single word properly, but just to know how to write the things we've been doing in class.
They are not overloaded because thay are usually given 5-10 words to learn.
I've tried everything, even those old-fashioned methods of copying a certain word 20 times. But nothing works, even games prove to be useless.

At times of despair, I think I'm a very bad teacher. After 3 months of teaching writing, the only thing they write properly are personal pronouns.

Is there any remedy? Do I do something wrong?

I would usually start out without writing. I'd play spelling games with alphabet cards instead, which makes it competitive and makes corrections very quick. Try to arrange vocab according to spelling rules rather than lexical groups. Add things like wordsearches and crosswords to get them used to recognising and writing words in a more controlled way.

Thanks for your reply. However, I've been doing all of these things and there are not results yet. What I could always do is be persistant.
I swear I'll teach them writing until the end of the school year.

Have you looked at phonics? Phonics And Synthetic Phonics - ESL Web Directory - UsingEnglish.com

I don't teach children, but it's said to help learn the system in English.