Front derailleur problem

[Sardine]

I'd be grateful for some advice. I've been having a problem with the front derailleur of my touring bike since I went on a 50 mile ride some weeks back which had a few off road sections (good tracks). I'd been riding the bike before then and also checked it over just before the ride and everything worked fine. During the ride, it rained for a bit and the off road sections threw up wet grit but not mud. The front derailleur then seemed to struggle with shifting down into the smallest ring even after being washed off with the help of my water bottle. It just kept sticking and wouldn't shift down without a fair amount of coaxing. I rarely use this ring but there were some long, steep climbs (for me) including one towards the end. Thankfully, it started playing up before I got to that point so I made sure I shifted down at the bottom of the hill in preparation - it's a well-known hill on a popular route.

Once I got the bike home, I cleaned it off, lubricated and re-adjusted the gears. The shifting was/is fine when I'm testing it at home (i.e no load) and most of the time when I'm riding on the flat but it sticks when I'm trying to shift down while applying a bit more pressure e.g. once I've started a climb. I went on a 30+ mile ride today and there were a couple of steep hills where it just wouldn't shift down. What do the mechanically minded among you think the problem is? My cable is in good condition but it's been on the bike for a while. Do I need to adjust the tension? I have to admit to being a bit obsessive about the cleanliness of my bikes and wash, degrease/relube at regular intervals.

Thanks.