The most likely explanation is that some dirt or grit has got into the gear mechanism.
Give it a really good blast with an airline and then use a silicone-based PTFE spray rather than an adhesive lubricant such as WD40: open the latch at the bottom of the main support and spray down into the gear mechanism.
Be aware that some of the PTFE spray may leak out and so it’s probably best to do outside and not on your best carpet!