I'm running QGIS 2.12 and have the same issue on 2.8, but it is now worse in this latest version. The issue is that not underlying raster layers (satellite photos loaded in as .tif) are being displayed. However moving the map, or changing the zoom level, can usually fix these particular layers but then others are not displayed.(see screenshots) This seems to me like a limit to the number of raster layers that can be shown at any time. However, this issue never occurred with older versions of QGIS.
Any suggestions how to fix this?
EDIT: Screenshots here: http://imgur.com/a/nzIT7