Making Shortcrust Pastry

In making shortcrust I normally use a 60/40 ratio of hard marg and lard to 200g plain flour.

This past few days I've tried to use butter and lard in the same ratio but find that it tears very, very easily. I work it the same way as I do with the marg/lard mixture. I'm just trying to master the butter/lard mixture.

Any advice on what might be wrong or on changing proportions etc

Kevin