Voltage regulation - Input voltage almost same as output
If the secondary of the transformer is supplying 20V AC you should have 20*sqrt(2) volt, being a little more then 28V, on the input of the 7824. That should be enough to make the stabelizer work correctly.  Something must be wrong with the 2,2 mF capacitor being 2200 uF. Replace the capacitor with a new one and try again. It is also possible that the transformer is not supplying 20V AC under load.That however is not visible from here.Consider also the voltage drop over the bridge. There is not much room left.