Where Are Miners?

The business has gone into full production. Now I spend about 1-1.5 million isk per day to buy material for future productions. This is the bothersome details come up. At the moment, I simply put up a buy order in the station where I manufacture my products. Tritanum buys very well. I am able to buy about a million a day without problem. However, more rare minerals take time to acquire. The one I want so much is Nocxium. One day I was sold a full order within hours. The next time, however, I waited for 2 day with barely any results at all.

What does this mean? I need to team up with a miners who can reliably produce minerals for me. I would be even willing to pay extra for this stability.