What were the downfalls or weakness of ancient Egypt?

Answer 1

There isn't really a single weakness that affected the Egyptians, there were many weaknesses to the empire.

The most well known failing was that the economy was founded on the grain which was supplied by the fields flooded by the River Nile. If the Nile over flooded or failed to flood sufficiently then the fertiliser rich soil washed down from Africa wasn't deposited in the fields and the crop would fail. This in turn, would result in famine. So one of their weaknesses was the reliance on a single crop.

The downfall of Egypt started with the conquest of what is now known as Alexandria in Egypt by Alexandra the great. By putting his general Ptolemy in charge he started a dynasty which ended with the death of Cleopatra the last Pharaoh and the annexing of Egypt by the Romans.