There’s something you need to understand. The rated mAh capacity of the battery pack is most likely rated at the internal battery voltage, probably 3.7 volts. However, the amperage measured by the meter is at USB voltage, 5.0 volts. To compare, you actually need to compare power, Watts, which is the product of voltage and amperage (P=VxA). So, for example 10,000 mAh at 3.7 volts is 37,000 mWh. At 5 volts, that’s only 7,400 mAh. (The actual power required to charge it will actually be a bit more, due to losses in charging.)
Bottom line, if you look at the charge power, it will be less than you expect, even on a full capacity pack.
