Unfortunately there is a design flaw with the A5 and A5S printers. The connector on the side of the bed is not sufficiently rated for the high currents required by the heated bed. Over time, these connections can start to heat up and oxidise, and when this happens the resistance of the connector increases. This causes a the heating and oxidising of the connector to become worse, creating a feedback loop.
This problem can result in the bed failing to heat properly. If you are suffering bed heating problems, you should inspect the connectors on your bed.
After becoming aware of this problem, JGAurora tried to improve the connectors they used on the printer. They replaced the 4-pin connector with a 6-pin connector. This actually caused addition difficulties, as the bed and the bed cable are often sold separately, so unless you replace both the bed and the bed cable, you can't connect up a new bed.
The second problem with their solution, is that some people have had the same issue re-occur with the 6-pin cable.
The type of connector JGAurora has used is not suitable. Instead, a high-current connection must be used. There are a range of different types that are suitable. RC Cars often have high current connections to their li-po batteries, so there are a range of RC terminals we can borrow from that hobby. e.g. XT30, EC5, Deans plug.
The wires themselves must be directly soldered to the heat bed to sure a good reliable joint, and then the wires cut at some point between bed and motherboard in order to add the connector. The wires must also be of sufficient thickness, AWG8 to AWG10 would be recommended. Silicon insulated wires help to ensure flexibility, but are more expensive.
Only the heating wires require the high current connection. The bed thermistor connection does not require such substantial connectors, and a standard JST-XH or similar connector can be used for that one.
Please see this article by Peter Nekomm (ufodoctor3 on the forum) who has documented his repair process.