G'day. Basic question if you would.

I've studied the basics of making bread, on this site, and been going OK. But the last three loaves have sunk in the middle. Any ideas of what variables I should be looking at.

Please understand that I'm a pretty rough and ready bread maker, even when at home. My main reason for baking bread is so I can make some fresh tucker when I camp out, traveling the Australian outback on an ATV (quadbike). No space for scales and a thermometer on those trips.