The most common problems are:
1. Different ram speed
2. Different timings
3. Incompatible Brands with the motherboard
this. also a quick look at the RAM's sticker would help you see if they are different in any way.
they must be exactly the same type/speed/timings/voltage.
verify that all RAM (if the same) are fully firmly inserted on their slots, just one a little off will cause problems.
lastly like another user posted run memtest.