Blog does not show all the way down to the More tag

My More tags are in the wrong place. I place the “More” say after 20 lines, but on the blog, only the first 3 lines show, then it says “more”. It looks like there is some kind of global setting “show the first 3 lines” which is over-riding the manually-inserted More tag. Is there such a setting, or what else could cause the problem?

Regards

Andy