Can be a number of things,pads seized in caliper bridge,I take it you lubed them with copaslip when you replaced them,caliper piston seized,try levering piston fully home then operate brake,may need to be stripped and overhauled,did have a seized front brake on a ford not long ago due to faulty brake hose but dont see that very often,make sure everything is nice and free and lubed up,good luck.
wickham
April 2010