Fountain pens need special care in terms of the ink flow mechanism.
Don't Leave Filled and Unused
It is best not to leave pens filled with ink, and not being used. That means if you have a number of pens, you will need a little discipline in terms of the rotation of your pens. If you are not going to use the pen, then empty it of ink, and give the pen a flush with water. Better to fill it up when you ready to use it and then use it, rather than spend time trying to unclog a pen where ink has dried up in the feed system.
Stand Pens Vertically
If you are going to store a fountain pen for a period of time, then flush the ink out of the pen, and pens are better if they are stored standing vertical. This means the nib is pointing up in the cap of the pen. What ink/moisture there is will flow down out of the tiny channels.
Flush on a Regular Basis
It is a good idea to give a fountain pen a good flush every month or so. This means, pulling in and pushing out the existing ink with good straight tap water.
I do it until no more ink shows in the water.
Then dry out the nib and ink mechanism by putting a ball of paper tower in a class and standing the pen, nib down. The paper tower will help to pull out the water from the nib and feed chambers.
Letting the pen sit vertically in some paper towel is a great way to draw out the water and any old ink from the pen.
Lukewarm Water Rather than Hot Water
Use lukewarm water, never hot. Hot water could impact seals in the pen.
If the pen is clogged, and after a flush of water there is still an ink flow problem, then I let the nib section stand in a glass of water for an hour or so.
I am always amazed at the trail of ink that flows. But do not do with pens made from rubber or casein pens (it looks like plastic but made from protein). Prolonged period of time in water could damage the pen.
Even after the nib is rinsed, as the photo to the right shows, if the nib section is left in a glass of room temperature water, within minutes streams of old ink will start to flow out.
Testing the Nib
If you have not used the pen for a while, and you go to write with it and no ink flows, don't just press harder and harder on the paper to get the flow going. You may damage the nib. Make short strokes diagonally across the paper. Use the same pressure as your write. The strokes will activate the flow. If nothing happens, then dip the nib in some water.