Through a loupe, the tipping looks like this.
The pen feels scratchy, and may skip in a particular direction.
The cure is to bring the tines into alignment (carefully!) using your fingers to (gently!) bend them.
Note that the misalignment may be due to the nib being off centre on the feed. Again, tweak the nib or feed (judiciously!) to bring it into alignment.
Poor gap set
Tines are squeezed together or too far apart. Pen has poor ink flow (too little, too much), may skip if the tines are too far apart.
The cure for too narrow a gap is to spread the tines of the nib. (see point 5 in John Mottishaw's excellent article).
Passing a thin (10 micron) brass shim between the tines can help with this problem.
Narrowing the gap requires removing the nib from the pen, squeezing the tines past each other so that they spring back to the correct position, and re-adjusting the alignment. Professional help is preferable, here.
Badly set feed
There is a gap between nib and feed.
Pen alternately skips and floods.
Cure is to reset the feed. Feeds are made of thermoplastics or hard rubber, and become flexible with gentle heat. Use hot water to soften the feed and set it up against the nib.
Manufacturing gunk in the ink system
Pen skips or doesn't flow at all because oil in the ink channels in the feed or in the slit of the nib prevents the flow of water.
I always assume this is there, and flush a new pen with water before use. Sometimes standing the pen in ink overnight helps.
The shape of the nib keeps the ink away from the paper.
Pen is a "hard starter". It writes well, but needs a bit of a push down on the nib to get it going. May skip a bit.
The cure is to grind away some of the tipping, to get a better shape. If you are at all unsure about doing this, seek a professional. I use a nail buffer, available for a small amount of money and a large sacrifice of dignity from the health and beauty section of the pharmacist. The finest grade buffer only!
It is best to work out what your problem is, before trying to fix it. Many of the symptoms are similar, and fixing the wrong problem may damage your pen.
A 10x loupe is sufficent to see these problems, though some prefer to use magnifications as high as 20x.
A nail buffer is not an emery board. It is a thick thing with three or four grades of grit, the finest being gray, and smooth to the touch. Women apparently use it to bring their nails to a high degree of polish.
Grinding the nib is a last, not a first resort.
It's also worth reading John Mottishaw's excellent article, Ludwig Tan's article on grinding italic nibs, and the articles in Richard Binder's reference page.
Arthur Twydle's article describes some of the more drastic measures taken in the past. I'm not a fan of spirit lamps and the like.
Edit: Added a notes section in response to some of the questions below
Edited by troglokev, 05 March 2011 - 11:36.