A few months ago the fobs on my '99 4Runner started getting funky and changing batteries didn't help. Locking was definitely more difficult than unlocking. Finally I took the fob apart and bent the spring contacts on both fobs to grip the battery more tightly. That fixed them both. Had to do the same thing to my garage door opener of about the same age a few months prior to that. That's how I got the idea. I think the contacts just lose springiness as they near the decade mark.