As a fountain pen enthusiast and restorer of antique fountain pens for over 20 years. I always tell purchasers that owning a fountain pen is like having a pet. Once inked they need to be used regularly. If they aren't to be used for awhile they should be emptied and thoroughly rinsed with water prior to storage. I also recommend against the use of cartridges suggest they stick to high end inks.
Of you find soaking in water doesn't work, try adding a teaspoon of ammonia to a cup of water and soak overnight. If that doesn't work an ultrasonoc cleaner with warm water will usually do the job.
All the above is true unless you have a nib tuning issue!