There are several issues that can cause this. A properly tuned nib should be able to write on first contact with the paper even after sitting unused for a week. You should not have to turn the piston knob once it has been filled. Gravity and capillary action is what cause ink to flow from a reservoir to the tip of the nib.
Common causes:
Clogged nib slit: This can be attributed to fast drying inks, old inks, bad quality inks (good brands can have a bad batch) and writing on inferior quality paper. (fibers trear off from the paper and lodge in the slit) The remedy for this is to first flush your pen, remove the nib and clean the slit. The safest and most non-evasive way to clean the slit is with a piece of brass that is .002 thick.
Too much taper in the nib slit: In this situation the the width of the slit from the breather hole to the tip of the nib is not constant but tapers to often to the point of the nib tips touching. The remedy for this is to press down on the nib at the breather hole with your thumbs and gently lift the wings of the nib with your fingernails of your index fingers.
Reverse slit taper: This is when the slit is wider at the tip than it is at the breather hole. Ink will tend to flow up toward tho hole as there is a lack of capillary action at the tip. The remedy for this is to push down on the wings of the nib thus closing the gap of the slit at the tip.
Baby's Bottom: This is when the inside edges of the tines at the slit are too rounded. When the nib is laid to paper and you look at the meeting point of the nib and paper, the curves resemble a baby's bottom. The remedy for this is to reduce the inside curve of the tines. Usually reserved for professionals.
This is just a brief and quickly put together post. For more detailed information I would suggest searching the web. Many of the professional nib meisters generally have a more intuitive write-up.
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