You might try flushing them out really well with something like WD40, or even under hot water and dish soap to make sure there’s no dried oil/lube in the joints. Sounds as though the joints just need a good cleaning and re-oiled.
Oil with a light oil, or gun oil. Work the tools back and forth. Type of oil to use is another entire topic of debate. I’ve found just about any type of gun oil, Ballistol, sewing machine oil, etc. to work fine for me. If it’s to be used around food stuffs, mineral oil is non toxic.
SAK’s generally do not have that heavy of a blade pull to be a nail breaker.