Anet A8 Hotbed Not Heating Correctly
That is actually not uncommon to happen for the Anet heated beds, many users report this. Mine was able to reach about 100 ℃ out of the box (took a while to get there), but not beyond.

For the bed to reach higher temperatures you would need to make some adjustments. You should at least insulate the bottom of the heated bed with cotton or cork to reduce the heat dissipation by half. Note to speed up heating to e.g. 110 ℃, I often insulate the top of the plate with a removable piece of cotton or cork to remove it just before leveling and printing starts. Furthermore, replace the heated bed connector and solder the bed power wires directly to the pins at the back of the connector. That connector is NOT rated for the amount of current requested by the heat bed! E.g. due to the moving bed the connector jitters which leads to sparks; you see many pictures of burned or brown connectors on the internet. While you do that please consider replacing the wires to the heat bed also to some proper gauge silicone wire, AWG 14 should be good enough.



WARNING!

Last thing to mention of prime importance is that the stock firmware of the Anet printers has NO thermal runaway protection! I.e. if your hot end thermistor would fall out of the nozzle block, the firmware would detect the temperature drop and will keep sending current even if it does not measure a temperature rise. This leads to an overheated nozzle and is definitely considered to be a fire hazard!



Edit:

The answers to question How to increase bed temperature over 103 degrees are of great value to this question.