Which Photoshop are you using?
If you go to the 'advanced' options for Brushes (where you can scatter, etc). Go to shape dynamics and just below Size Jitter there's a drop down. Set it to Pen Pressure when you get the problem.
If it's already set like that when you have the problem... then... I don't know. D: Make sure that your computer detects the tablet (no more bubbles notifying you of the tablet at the bottom right) before you open up Photoshop? o__o
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