I haven't had one for a while but a squirt in the general direction with some WD40 always used to do the trick. I never understood why I lubricant would help but it did seen to work

I'd advise against using WD40 on a friction belt - it contains oil!

Try water instead, It'll evaporate.

Maybe the oil attacks the rubber, rendering it sticky? Short-term fix, long term damage?

Here's where I suggest my automotive panacea:......wait for it.......SUNFLOWER OIL.

It'll go sticky itself, and will be less aggressive to rubber.

But I SUSPECT it does eventually soften rubber too, so long term it might not be so great.

I have heard of commercial "belt dressings", but I dunno if they are any good, or what is in them.

I treasure a decades old yellow can of Grako Sprazon belt dressing by Graton & Knight. It doesn't say what's in it but all my cam and fan belts got a squirt every year or two until recently. None were ever changed, none ever broke. One, a VW cambelt, was still going strong at over 240,000m when sold and my nearly 40yr old TR7 fanbelt is still functional (but I carry a 38yr old spare and a couple of spanners!). On the can it says it's suitable for all types of belts from fabric to synthetic, but I'm not sure about the modern aramids, so our 19yr old Focus cambelt hasn't had a squirt recently. But, as said, check the pulleys it drives aren't stiff and beginning to seize. Feel how hot they are after a short journey.