This happened to me last year while doing alot of enduro riding!
I found it was the brake pads which were of a compound that needed to get up to a high temperature before they would start to bite correctly in the cold.
I changed them for a good intermediate compound and sure enough i no longer had to pull on the lever so hard for them to bite.
It could be a number of faults though,personally i would bleed the system and check the seals first as fluid is cheaper!