18650 cells - Unable to measure capacity
It basically means that your cell samples are at the end of life and have developed substantial internal impedance. Apparently your tester is an intelligent one, and cuts off the discharge process at 2.75 V (or it could be the cut-off from internal cell protection, doesn't matter). When you set the current at 500 mA, the cell drops its output below the cut-off threshold, and the tester times off the capacity at very low value. 

When you load your cells with 100 mA, the internal voltage drop is smaller, and cells last longer before the threshold is reached.

You need to perform a simple load test. First, measure the no-load voltage first, say V1. Then connect something like R=20-Ohm resistor. And quickly measure the resulting voltage, say V2. Your current is V2/R, and the internal impedance would be R * (V1-V2)/V2. If the result is more than 0.2-0.4 Ohms, your batteries are no good. It looks like in your case the internal cell impedance is about 2 Ohms (~1V drop from 500 mA load).

I am afraid your batch of batteries is bad, and you need to dispose them.