I like to use a heavier oil in my treadle assembly than sewing machine oil. Oil everything that moves including the axle for the power wheel, The hinges for the foot base and the pittman arm, if it is iron.
You need to isolate the head from the treadle to figure out where the drag is.
Will it give you good stitches if you turn it by hand?
Some folks pracrice treadleing without thread to get the ryhthm. I've heard if you can carry on a conversation while you treadle, you have it down.