I guess its giving mixed messages to sometimes have a wee on the toilet and others (out and about for instance) to go in a nappy? Do children get confused or can they distinguish 'the right time and place'? 
What do you do at the beginning of toilet training when its very very hit and miss? Sometimes a loo is just not available and we are in a situation where (2 weeks in) where asking for the toilet hasn't been learned yet. Today was the first successful day with 2 wees on the loo! 