Hi,
The "stick-on" style gauges work by detecting the temperature difference between the gas and liquid regions of the bottle.
When you're using gas (at a moderate or higher rate) the liquid boils and sucks in heat from the outside - so the bottle wall cools down - and you may even see droplets of condensation on the bottle surface - exactly like a really cold beer...
When you're not using gas - the whole bottle reaches one even temperature.
So:
1. If you're not using gas - the gauge doesn't work.
2. If you are using gas - then the gauge does work - but you may be able to see the level from the condensation anyway - or just running your hand up the bottle and feeling the bottle warm up will indicate the level just as well.
Suggestions:
1. A Gaslow (or similar) pressure gauge is a reasonable indicator. In truth they only indicate when you are about to run out...
2. A BP GasLite bottle with translucent walls is the easiest to use - you can see how much gas is present
3. Or place the bottle on a cheap set of bathroom scales - as long as the securing strap is loose and the pigtail is arranged sensibly (so that it doesn't cause the scales to misread) - then these give a good indication of gas content. Sure you need to subtract the weight of the empty bottle - but they do work...
Robert