Just looked up some data from my underfloor heating manual. For 100W/m2 emission, from a cement based floor, with ceramic tile over, the surface temperature should be around 28C, so you are low. The type of tile cement is critical for performance, as mxslick says, there are latex-rubber based flexible versions which will give poor heat transfer- a cement based product is best. An under-element insulation layer is vital, ( I bet this is your problem), or you will be warming the worms, [a few inches down, the earth's temperature will be 10C (50F), a massive heat sink].
My set up is hot water-pipe based, with a design heating of only 50W/m2, (wood floor, cork tiles over). The design floor temperature is only 23C so we are fitting supplimentary electric towel-rail heaters in each bathroom, using the floor as background only. If you want keep the aesthetics of a room uncluttered by radiators, rails or radiants, I'm afraid lifting the floor tiles may be the only option, since it seems you may need to put in a better insulation layer under. Also, for any underfloor heating to work properly, due to low emmission power per sq foot, the wall/ceiling insulation has to be really good too, R = 2.5 (metric) minimum = 100mm (4") of glass-wool minimum. Better is the new reflective foil multi-layer stuff, 7mm (0.28") thick is supposed to = 6" of rockwool but it's expensive. One thing all underfloor systems have in common is they don't work well in excessive drafts, ie. a ventilation fan tends to spoil the effect by dragging what little power you have outside to heat the sparrows! Since my guess is youre DIYing this project, get the floor tiles up, or improve the insulation and/or fit a heated towel rail and put this down to a learning experience, but leave the electrical stuff strictly conventional.
Stay safe,
Alan
