Don't ever increase the amperage rating of a factory fused circuit that is already in place, the circuit may not be able to handle the additional draw and a fire will occur.
Is the bulb OK, sometimes during shipping the bulbs get damaged a little, that is enough to fail shortly after install; Check it !
If the bulb is OK then look at your wiring, pull it out from where it is attached and look it over, don't tie it in place until after the system id working OK.
Double check the ground, is it a good connection to bare metal ?
Add External Star Lock Washers to teh connection between the Ground Lug and the bare metal.
Take a look at where you connected the dome wiring to the Cab Lighting, look for poor connections, poor soldering (if you soldered).
Solder all of your connections using the newer solder with a water based flux, soldering will almost garrantee the connection to be solid.
Test the overhead light out of the new circuit to verify it is not the light.
Not sure where to go from here, you don't usually need a relay for overhead lighting but if all else fails you could try to have a relay under the dash. Hook the control of the relay to the Cab Light circuit, then run power from the battery to the light using a fused line.