The single problem that is seen the most often on GCs is an overtightened septum. Proper tightness is achieved by screwing down the nut until the "E" clip on top stops moving --- and then only about 1/2 turn more. Any tighter compresses the septum vertically and it will actually tear out more when the needle pierces it.
When the plate (green arrow) touches the septum, the spring washer (yellow arrow) starts to compress and the "E" clip lifts up (red arrow). There is only about 1 mm of available compression space before the spring washer bottoms out! When the "E" clip has lifted up 0.5mm or so - about the thickness of a 26 gauge syringe needle - STOP! That is tight enough to do the job. The septum only needs to be held in place and barely compressed by the spring washer to make a seal sufficient to hold up to 100psi.
If you think you need to tighten it more to get it to seal, replace the septum and start again.