Probably a simple question for this crowd. 

First the setup:  approximately 900 sq ft of a 1000 sq ft basement heated with a navien 240a and Heating box (and forced air).  This unit is not modulated but i set it myself and currently with the milder climate (below freezing at night, above freezing during the day) the water going into the slab is about 38C.  The thermostat is a Tekmar 508 with floor sensor set to floor sensor only.  We have the t-stat set at 19C.  The sensor is in the middle of the entire slab but on the edge of a radiant heated area next to an unheated utility room.  The sensor is about 3" below the surface of the tiled floor in a drilled hole (ie easily retractable).  Slab is about 5 ft below grade and about 6-8" thick. 

Not an ideal setup but this is what we have to work with.

Problem.  We set the t-stat and the radiant system warms the floor.  Feels great as the equipment works to satisfy the t-stat.  One the t-stat is satisfied, the unit shuts down and it doesn't run again for a day or two.  Meanwhile after a half day of not running, the floor surface is not feeling like it is warm anymore but the t-stat is showing that it is still at 19C.  

Questions.  Is the floor sensor too deep?  Is there that much of a temp gradient between the middle and top regions of a cement slab?  Could i correct this problem by drilling a hole closer to the surface, ie just under the tile, and inserting the floor sensor there?  I have access to the cement floor profile in this location so this is doable. 

Any suggestions?  If this is not enough info or not clear, please let me know

Thanks.

First I need to admit that I know nothing about using a floor temp thermostat to control the radiant system. But your post has me interested and the following questions popped immediately into my curious mind. Have you measured the temp of the slab at the surface to see if it really is any different? Is 19 C really the surface temp you want? A 19 C slab would feel pretty cold to me. Hopefully some experts will join in.

why are you not using the air sensor as well?

I don't use the ambient sensor because we have a forced air furnace that also serves the basement.  If the furnace was on, wouldn't it have the effect of satisfying the t-stat therefore shutting off the radiant?

Currently the floor setting is 19.5 and it feels comfortable.  That is, it is 19.5 where the sensor is, and the sensor is on the edge of a heated zone.