Make sure the screw on the head of the cutter is on top.
Use white spirit....dip the cutting wheel in white spirit every 10 or so scores.
Press the cutting head down on the glass so the small spring between the cutting head and brass shaft is completely depressed, then add more pressure to create a score.
Keep the score line even and unbroken.
Snap the score line.
Second hand glass is harder to cut than new as glass hardens with age....
Cutting glass takes practice....use 3mm clear glass / horticultural (greenhouse) glass for practice.
With coloured glass / textured glass always cut on the smooth side.
Golden Rules...never score over a score and try not to share a cutter with anyone else.