Open windows system settings and check how much RAM is actually displaying. If it isn't showing 16GB, then one of your modules might be faulty?
If so, also check the BIOS if its picking up each individual RAM module. Test each RAM module by plugging them in one by one and see if one might be giving an issue?
And in task manager check the start up tab for programs that you don't need during windows boot. Disable them. This will also reduce the amount of memory needed for boot up.
Hope this helps!!