Stunted growth?

3 years ago I planted some Veronica Atomic Pink ray, it was great the first year, but the last two summers the growth seems very stunted and although they do manage to flower, they certainly aren't flourishing. 

We are on light sandy soil, would this explain them being stunted, or would it be not enough sun ?  Would it be worth me moving them to a sunnier part of the flowerbed, (although, they do currently get a fair few hours in the summer)

I'm considering digging them up, mixing up the soil, adding in more manure and putting them back in or doing them same and putting them somewhere else.

Any ideas ? Thank you.