question about lop-MAP algorithm

hello,
I want to ask a question about lop-MAP algorithm.
I wrote a c program about a decoder using lop-MAP algorithm.
When I use only one decoder and there is no iteraron, to some inputs,
it can decode very well but to some other inputs it has very high bit
error rate.
I don't know whether it is because of not using iteration.
and I also want to know whether it is normal to have very high bit
error rate when decode the inputs only once(I mean withour iteration).

looking forward to your answer 


thank you