You know that when you leave your ink bottles unused for a period of time, the pigment and binder may separate. And mixing them completely again is sometimes not easy because the pigment has stuck to the side of the bottle.
This is a problem I have all the time with my Dr Ph Martin's Coloured India Inks.
The quick tip to solve this problem is to go buy some stainless steel ball bearings and drop a few (maybe two) into the affected bottles. This is basically the same trick used in paint markers. And now there's an agitator (or two) inside to shake things up.
It's important to get only stainless steel ball bearings so that they don't rust. The alternative is to get plastic ones.
These are the 4mm ball bearings that I bought on eBay for less than US $10 inclusive of shipping. It's really worth the money. 4mm is a good size that can still reach the corners of the bottles, and heavy enough to shake easily.