It sounds as though the angle you are using with the brushcutter is too great for the fuel mix to reach the engine - and perhaps you are using it at an angle for which the machine was not designed.
If you really need to use a brushcutter at odd angles, take a look at the EM2651 from Makita. This has a 25.4cc four-stroke engine with full wet-sump lubrication so it will work at any angle. There is a choice of bike-style or D-handle and the latter can be fitted with a hedge trimmer attachment. Both machines have a mechanical decompression facility to make pull starting lighter and engine starting more efficient.
Alternatively, try the new battery-powered models from Makita and Stihl or the Pellenc equipment supplied in the UK by Etesia. In all cases, however, you should observe the correct stance for brushcutter operation. Using such equipment incorrectly can cause injury.
For brushcutter reviews, listings and buying advice, see whatkit.co.uk.