How short is a "short time". If there was a promise on the tin and that hasn't been met you may have a claim against the manufacturer. Full gloss paint tends to yellow with age due to the linseed oil content.
However, how did you prepare the surface? Did you clean down the original paintwork with sugar soap first? Did you rub down and undercoat before painting?
If you have nicotine stains, greasy fingerprints, dirt etc and paint straight over it then it will bleed through in a very short time indeed.
mizzy, Most paints that contain linseed oil will yellow when exposed to air and if not directly in natural daylight, sunlight, which bleaches the linseed oil in the paint. This can be seen on window cills and door jambs, the window cills will stay white and the door jambs will yellow also it depends on what brand of paint you use some of the cheap brands of glosses contain more of the cheap pigments (extenders which are naturally opaque in oil, china clay, lithopone, barites) as opposed to the more whiter pigments such as (titanium, white lead, zinc oxide) the more of these types of pigments the whiter the paint. A cure to the yellowing is to use a water based gloss such as permoglaze mvp which is one of many water based gloss on the market, the only problem you have to be fairly quick at applying it or else you will get brush marks this can be over come by using a long bristled brush and a soft foam roller for large areas.