It has three possible causes:
First of all, the tip of pen may have been worn away. Or check if the flow-channel of tip is not clogged with fibers of paper. To remove clogged fibers, pull away the tip from the pen and wipe gently with paper towels. Also when the writing pressure is too strong, it will cause the tip to open too wide and become irreversible.
If you feel the drawing lines has became wider, use the new pen.
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