Changed some components now PC wont boot: DRAM fail light on

I built a PC with some spare parts and installed OS & programs
Son brought his old PC over and I swapped his RAM, Cooler, PSU and GPU.
Now PC wont boot.

CPU light comes on goes off a few times and stays off
Ram light goes on/off a few times then stays on.

Temp PSU: Corsair RM750x
Sons PSU: Corsair HX750i

Temp GPU: GTX 1660 Ti
Sons GPU: RTX 3060

Temp RAM: F4-3600C16Q-32GTZN
Sons RAM: F4-3600C16D-16GVKC kits x 2 for 32GB

Temp cooler: Corsair H115 RGB Pro 280mm
Sons cooler: Corsair H110i V2 280mm

With the temp parts in system was fine.
After installing sons parts wont boot.

Put temp ram back in. No go
Put temp GPU in. No go
Tried with a single ram stick. No go
Tried with various combinations of 2 sticks. No go
Reset CMOS. No go.

His old system was working fine before he brought it over.

The new motherboard has 2xATX power connectors where as his old ASRock Z390 Phantom Gaming 6 only had one so I had to grab a spare Type 3 cable to add to the sons PSU. The RM750x uses different cables.

Tried one CPU both ATX cables then one at a time. No go.

CPU, Motherboard and case are brand new. Boot drive is Kingston SC3000 1TB Windows 10 21H2. Working fine until I swapper over parts.

Tried RAM from a third PC. No go.

I'm out of ideas.

Replace the motherboard.

Nah... it's brand new.

Had a couple of tries to get it to boot, eventually got it to boot.

Flashed the BIOS and good ever since. One of the recent bios's addressed some RAM compatabilities, so that's probably what fixed it.

Ran Memtest for a couple of hours. No problems with RAM. Tried RAM in another PC and it worked fine.
Ran some benchmarks to put it through its paces and so far no more issues.

Why it refused to boot for the best part of 2 hours is beyond me, but it seems stable now.