The easy way to test this is to set the weight on the scale to 0 and if the arm floats, it is correctly calibrated. Set it to 1.2 for playback. It could be that the suspension on the cartridge is failing. Try warming it up with a hair drier and see if this makes any difference! Not too hot!

Having agreed with Peter, I would add that the last time I had a similar problem, looking at the stylus under the microscope revealed a horrifying mess: deposits which I was sure that careful record & stylus cleaning made impossible! alcohol & a camel-hair brush restored the sonority of sibilant sounds.
I usually use a test record (try e-bay), in my case Hi-Fi Sound 75, to set the stylus pressure by ear, as the scales can be inaccurate on arms, as can pressure gauges - we are trying to judge fractions of a gramme, with static & dynamic friction to take into account, not to mention our judgement of the 'floating' state. If you look back at the Hi-Fi magazine tests of the 70s, the 'optimal' tracking force wa often at variance with the cartridge manufacturers' claims - one may suspect that, since low tracking force was seen as desirable, optimisic claims may have been made by some of them, like acceleration times for cars!
Record wear may lead to a requirement for a less sophisticated stylus profile & a higher tracking force, too. I use an MMC-2 tracking at 1,3g for 'very good' discs, & an MMC-3 at 1,75 for worn ones.

Does the RX have this? I know some 5000s do and the RX2 does, but I didn't know if the RX did.