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The first time, the igniter (manufacturer provided) couldn't burn the motor fully. The second time we used a Widmann igniter but as soon as the ignition was provided, there was some spark at the nozzle side (we thought the ignition was started) but then the ejection charge went off less than the delay that was set to it (8 secs) on the launch pad itself popping the upper stage and the booster section caught fire and burnt itself completely. Video attached for better visuals.
https://youtu.be/ztDF4deKVMg
Our final conclusion- The motor casing and the motor where not coupled in completely and had a ~2 mm gap in between. Hence when the chamber pressure was built up after spark, the ejection head had a chance to open up with that gap and thus the plume traveled upwards activating the ejection charge and burning the booster section of the rocket.
