The typical hack for this is to put the dough in its covered bowl into the oven and turn on the oven light - just the lamp; this puts out enough heat to warm the inside of the oven to about 30° C if you leave the door open a bit. However, our oven is a shiny new-fangled thing with a light that goes out after about 15 seconds... so I needed another way to gently heat it up.
The solution? A candle or, in my case, an oil light. There was a bit of trouble at first, though, as the light went out after a short while - and it took me a little time to discover the reason. Which was... old oil on the swimmer.
When I sell these, I usually tell my customers that if they are not using the light for a while, it's a good idea to take the swimmer out and give it a good rinse to clean it, then dry it.
Very obviously I'm not always taking my own advice... this swimmer has been in use for a rather long time, with breaks inbetween uses where I sometimes took it out and rinsed it and sometimes not. Over all this time, a good layer of gummified oil has accumulated both on the metal bit and on the cork.
That is, unfortunately, not just an optical issue - the gummified oil warms up when the light is burning, and then seems to get just liquid enough to get into the wick, blocking it up. Which means that the wick cannot get enough oil through anymore, and the flame goes out.
So - should you have an oil light that does not work properly anymore, check it for gummified oil; clean it if necessary (hot water and soda, and a good long soak in there, will help), or get a replacement swimmer.