Underfloor heating's generally not a great idea to be quite honest. I wouldn't use it at all if avoidable.
The heat output entirely depends on the make up of the floor and the floor covering. For example, you could be loosing most of the heat into the foundation of the house if the concrete isn't holding much heat.
Also, as suggested the tile adhesive or the tiles may be insluating the floor too making the heat transfer pretty poor. (particularly if the adhesive is put on very thin behind the tiles as you effectively create an air cavity insulation chamber!
Other reasons for not installing underfloor heating:
(1) Under wooden floors it creates a lot of problems as it tends to dry out and warp the wood. It can shrink, get brittle, change colour etc over time.
(2) In bathrooms, where it's usually used, it can create horrible smells. If the bathroom floor is in any way not 100% clean it can really create a whiff. Particularly where a toilet is present. i.e. any splashes will be evaporated.
(3) it's woefully ineffective in many cases and very unreliable.
(4) if it goes wrong you need to dig up the floor!
Personally, I think just heat your bathroom conventionally and don't rely exclusively on underfloor heating. It's a nice gimic to create warm tiles but, that's about it.
