RAM must be installed in same size pairs from inside out. If you have 4 slots, the pairs are installed 2-1 1-2.
If you have 8 slots, the pairs are installed 4-3-2-1 1-2-3-4.
Additionally, if RAM is installed correctly and if the RAM is, in fact, guaranteed Mac compatible RAM (not PC value RAM), solder degredation may be the cause. Though typically affecting startup as well, RAM not reading has been reported.
Heating the logic board around the RAM slots with a hair dryer is a way to test this.
The area must be heated to hot to touch, then start the machine.
Be sure that RAM in use is Mac compatible.....