Oil filter keeps on leaking. Why?

Hi, I have just done oil change on my landy, 2.5td 19j. Put on new filter, ran engine and after a while oil started running down filter on to floor. Checked it was tight, could`nt turn it any more.
Thought it might be defective, bought another, both Cooper, and second one is still the same. Never leaked before, so whats the problem. Any ideas.
Ta

Did you smear the seal with fresh oil before fitting the filter?

you haven't accidentally left old oil seal on, failing that faulty batch

Yes,and part filled filter before putting it back on.

Not that I has noticed.

If it is a spin on filter I have had fram and crossland ones leak on seam

It is a screw on type. But have never had leaks before.

I stopped using Cooper filters on my 300TDi for this very reason, the leak was never very significant but there are no other leaks on the vehicle and I didn't see why I should put up with this from the oil filter. I tried three Cooper filters but they all did the same but have never had any problems with the genuine & Britpart filters I have used since.
Used to use Cooper fiilters on the 101 and they were fine.