Computer powers up but won't start (beeps)

I just built a new computer yesterday and I'm having a rather strange issue.  I'll try to post as much detail as possible:

The motherboard is a Z170 Pro4S and I have 16 GB of RAM (2 sticks of 8 GB).  After I was done putting the system together, I powered it on and went into the BIOS.  Everything seemed fine, the CPU, HDD, optical drive, fans, etc. all showed up, except only one of my two memory sticks showed up there.  The RAM brand/model is Corsair Vengeance LED (DDR4-3000), which means the sticks light up.  For what it's worth, both sticks lit up so that's telling me they were at least properly seated and getting power.  So I turned off the computer and tried re-seating the RAM in different slots.  I was using A1 and B1 so I moved them to A2 and B2 and rebooted.  Then I couldn't even get into the BIOS, as I was getting three long beeps upon powering on the system.  Everything else seemed to be running, the video card, fans, hard drive, optical drive, etc.

I tried swapping the sticks, using different slots, I even had to reinstall the CPU fan because it's a bit flimsy, just opening up the RAM slots sometimes causes it to pop out.  Either way, I even reset the CMOS (not that I think it was needed but anyhow) but it made no difference.  Every time I powered up, I got the three long beeps, and almost every time, I get a different combination of: one RAM stick lights up but not the other, neither of them lights up or both of them do.  I'm 99% sure the issue is either the motherboard/memory controller or the RAM itself but I don't have any other computer to test the RAM on since my other system is 8 years old and only accepts DDR2.

Any ideas?

By the sound of things I'll say you're not seating the sticks FULLY into their slots.

You should hear 2 clicks when they seat.

Hi guys, thanks for your replies.

Stefan75: Yes, I tried with one RAM stick at a time, with both of them but nothing changed.  I still had the same problem.

parsec: I'd be surprised if the DIMMs weren't seated properly, although nothing would make me happier if that was the problem.  Strangely, the Z170 Pro4S only has one clip, at the top of the slot, to hold the RAM in place.  Every time I inserted the sticks into the slots, I made sure that they clicked and the clips weren't open.  Is it possible that the DIMMs aren't properly seated even if those two conditions are met?

The motherboard's instruction manual mentioned that slots A2 and B2 should be populated first, and then A1 and B1 although I'm not sure if that makes any difference.  Either way, they should at least be detected by the board.  If the memory controller is part of the CPU, then that eliminates the possibility that there's a faulty controller on the motherboard.

To answer your last question, it's the entire fan/heatsink setup that moved when I opened the locking tabs.  The pins holding it in place would sometimes pop out but the fan itself is solidly attached to the heatsink.