Many dials become sluggish after a while, usually through dust and/or over-lubrication in the past. If you are lucky they will respond to a very small drop of light oil on the insides of the governor cup and on the governor worm drive. DO NOT use WD40. If this fails then you will have to do a full disassembly job (tedious!), followed by washing in white spirit. Afterwards lubricate the re-assembled item with a minute amount of watch oil. Maplin and other electronics component shops sell a micro-spout oiler containing oil and Teflon particles for a very moderate price.
If the problem is a broken spring, this advice from Steve Hilz will help. You can rewind dial springs by carefully pushing the mainspring into the holder and working it into the centre as you go. You have to watch out that you don't let the spring release, but it will go back OK with some patience. Wear gloves so you don't cut yourself. If you have a broken spring near the end, you can anneal it in a flame and let it cool gradually. Then, make the bend that you want to be permanent. Then, reheat the portion of spring that you annealed, until it is red, then quench it in cold water to restore the temper.
Usual dial failures are:-
Last revised: May 08, 2019