Front Mech question

I've got a hybrid with three chainrings. Just recently I was on holiday and riding up some steep hills in Sussex and I started having trouble changing into the small ring - it would just make a bit of a grinding noise, but not actually change, so I'd have to keep going in the middle ring. At first the problem only happened when I was going up steep hills and trying to change when I was already on them (i.e. already piling on the pressure) - if I tried to change on the flat (with much less pressure on the pedals) it was OK. This was a bit annoying as it meant I had to change down into a really low gear before I'd even got to the start of the climb.

I fiddled around with the limiter screws on the front mech, and made sure I was adjusting the right one in the right direction, but I couldn't actually fix the problem even when I made a massive adjustment. In the meantime the problem has got worse and can happen on the flat now as well (but, oddly, doesn't happen all the time, just randomly).

Has anyone got any idea what the problem could be and how I could fix it? I wondered if the chain might need changing (I have no evidence for this, though, it was just a wild guess). I've just been to a bike shop, but the bloke there didn't seem especially interested. He looked at it for about ten seconds and said that the cable was too tight and he slackened it off a bit by turning the thing where the cable joins to the changer. I won't know if this has done any good till the weekend when I get another chance to ride.