Hi I get this problem quite often.
I open an older scene prior to when denoiser was introduced and add denoiser in elements.
Hit render but no denoiser elements and effects are rendered.
To get the denoiser to work I have to merge everything into a fresh scene.
Maybe it’s the same problem as mine where denoiser doesn’t work on low camera f and shutter speed settings.
Guys at Chaos also suggested to increase the post effect rate (under settings tab). Default is 10. In my case that didn’t help, so they also suggested to use 4-6 f number and shutter speed 40-80. That helped in my case.
Do you get any warnings in the log ? One possible reason for the denoiser not to work on old scenes is the old adaptive sampling method.
You can change it with the following MAXScript command: renderers.current.twoLevel_adaptiveMethod=1