What have I done wrong when replacing tap body?

Hi - both my kitchen taps were very very stiff. I dismantled them and took one of the bodies to STC. They sold me a tap conversion kit. I have fitted these bodies and heads. If I tighten the body with a spanner then fit the head, I cannot open the tap. If I slightly loosen the nut on the body and re-fit the head, I can open the tap, water comes out, but it takes a while to close the tap (I know this is not a solution as the nut should be tight). Looking at the inside of the heads, there are ragged holes in the inner shells. I tried instead to fit my old heads (which are the same as the new ones), but I still have this problem of not being able to open the taps when the body nuts are tight. Anyone have any ideas please? Thank you.