Did you extend the motor tube (BT-50) up to the nozzle of the upperstage engine, if this rocket used a larger size body tube than BT-50?? With gap staging, you really need to have a stuffer tube to contain all the burning particles of propellant and hot gases (there is a HUGE debate as to which actually does the ignition part of the upperstage, or infrared radiation from some combination of the two) so that the blow-through of the lower stage engine is 'aimed' down the 'barrel' right at the upperstage engine nozzle.
You did the right thing with the ports to relieve the pressure, but if the engine tube in the booster is short while the booster is larger than the motor, there's a lot of space for the heat/particles from the booster engine to get 'lost' in and dissipate to the point the upperstage won't ignite...
Also, it's always a good idea to check the nozzle end of your upperstage motor to make sure that NOTHING is blocking the nozzle... sometimes a spare bit of clay from the nozzle forming process when the engines are made will be partially embedded or covering the end of the propellant grain (the "dimple" that the ignitor goes in) and anything that gets between the propellant and the blow-through from the lower stage motor will greatly reduce your chances of upperstage ignition.
Hope this helps! Good luck! OL JR