Ink is too pale (Make sure to let the ink oxidize after use to judge the full color)

Too much water added

Allow excess water to evaporate

Too little tannin extracted due to errors in preparation 
/stale galls

Add aditional tannic acid

Too little iron sulfate

Add aditional iron sulfate (may reduce the stablitiy of the ink)
OR add dye or lampblack pigment

Ink does not flow well out of pen

Too little gum arabic

Mix in liquid gum arabic

Too much gum arabic

Mix in small amount of vinegar

Incomplete filtering

Filter or decant ink from settled solids

Exposure to air causing excessive pigment formation in inkwell

Filter or decant ink from settled solids

Ink becomes moldy very quickly

Warm environment. Organic contaminant present. Container not cleaned thoroughly before use.

Skim off mold and place in refrigerator to slow mold growth. Traditional cures call for added vinegar, alcohol or salt.

Ink is too thick

Too little liquid. Too much gum arabic.

Traditional advice suggests adding vinegar instead of water to dilute.

Black sludge forms at bottom of inkwell

Container is too large, allowing exposure to air. Container left open.

Blow into bottle and quickly replace cap to replace oxygen with carbon dioxide. Seal container when not in use. Filter and transfer to smaller container.