Fountain pens can sometimes misbehave, even new out of the box. Perhaps the most common problem with newly-acquired pens is when they do not write, or write with insufficient ink flow. In almost all instances, the cause is an oily deposit which prevents proper capillary action in the nib and feed. Note that this is not always a case of manufacturers failing to clean out their products properly, for many pen materials give off vapors which will eventually leave a microscopically thin layer of oily condensate on nearby surfaces. With a new pen, filling it with cool water with a small amount of dishwashing detergent added will usually solve the problem (be sure to clean out with plain water afterwards). With a vintage pen, only the nib usually needs to be dipped in the degreasing solution.
Many cases of irregular flow can be traced to improper filling technique. It is always a good idea to consult our online instructions, as sometimes there are quirks that are not intuitively obvious -- such as the need to leave the plunger depressed when removing a Vac-filling Parker 51 from the bottle after filling. The most common mistake with pens with ink sacs is removing the nib from the ink too soon, or not fully immersing the nib up to the rim of the section.
Figuring out why pens drop ink can sometimes be difficult. Often, however, it is simply a matter of the ink level being very low. Ink does not change volume very much, whether subjected to changes of pressure or temperature; air is another matter, and when the ink reservoir is full of air, ink flow is bound to be less consistent. Another common cause of dropping ink is an ink sac that is too large and floppy. Installing a smaller sac will often remedy this problem. Old sacs (or diaphragms, in the case of Vac-fillers) may also develop microscopic holes too small to permit ink to escape, but large enough to allow air into the ink chamber. This will inevitably lead to dropping ink.