You need to increase the power of the heated bed. With a given amount of power, there is an upper limit to the maximum temperature you can reach because at a given point losses due to conduction, convection and radiation will balance out the heating power and the temperature will not increase any more.
Sometimes, inability of the bed to heat up is due to the supply voltage sagging under load. First, measure the supply voltage with and without the bed turned on. If you find the supply drops significantly when the bed is turned on, you need a new power supply.
Otherwise, you will need to either:
Get a new, higher-power heated bed. Make sure that it is compatible with your electronics, or upgrade them as needed.
Increase the supply voltage so that the bed you already have will give more power. Some power supplies have a small adjustment potentiometer that lets you adjust the output voltage. Be careful when doing this. Even a small change in voltage gives a big increase in power. For a heated bed with resistance R at voltage U, the power dissipation is U2/R. Going from 12V to 13.5V already gives 26% more power.