The stick that it doesn't boot with is most likely broken. Use Windows Memory Diagnostic or Memtest86 to test it. Try both sticks, then one stick, then the other. Whatever stick doesn't pass is broken. it will tell you if there is an error or hardware issues.
Open ya MoBo manual..... look for the page where it shows ya where to put the sticks......put 1 stick in the slot it suggests for 1 stick..... run memtest86+ for 2 hours this time around...... switch sticks...... if they both pass, look where manual says 2 sticks should go. Run memtest86+ overnight