You have already said that you are getting 11.99V across the heatbed terminals and that you are using a 12V power supply. This means there is nothing wrong with your mosfets or cabling. The problem is that at 12V, your bed heater is not powerful enough. You can solve this in 3 ways:
1. Improve thermal insulation.
2. Increase voltage.
3. Replace the heater by a more powerful one.
See my previous reply for some suggestions regarding the first two. In your position, I would replace the PSU by one that you can turn up to 14V. Heater power increases with the square of the voltage.
I am assuming that firmware is not causing the heater power to be reduced to soon. You can verify that by commanding a very high bed temperature and seeing whether the time to get to the temperature
yiuvactually want improves significantly.
1. Improve thermal insulation.
2. Increase voltage.
3. Replace the heater by a more powerful one.
See my previous reply for some suggestions regarding the first two. In your position, I would replace the PSU by one that you can turn up to 14V. Heater power increases with the square of the voltage.
I am assuming that firmware is not causing the heater power to be reduced to soon. You can verify that by commanding a very high bed temperature and seeing whether the time to get to the temperature
yiuvactually want improves significantly.