The soil of Santorini is enriched in C12, and this has an impact on the RC dating of stuff on the island, including plants grown in the soil (samples from still-living plants having a much higher RC date). Seems like thermoluminescence, electron spin resonance, and perhaps other forms of radiological dating would be a better choice.
Sounds more like the soil, or some components of it, are deficient in C14. I know limestone can do this, but I am not sure how volcanic soils would do the same.
I would be curious to know that the C13/C12 ratio is, as that is often a good indicator of problems with C14, and it reflects on the material you are dating.
This is the kind of thing you have to watch out for in C14 dating.
Normally we date several different materials and compare, when possible, against other dating methods.