>>1451481feel all alongside the inside of the tire and see if you can feel anything rough for a start.
Also check the rim tape and inside surface of the rim to make sure no nipple heads etc are poking through and there's no debris in there.
If you align your tyre and tube the way anon and i are telling you and it's puncturing in the same place every time, it should be very easy to find the source of the problem.
One thing I've had happen but is kinda rare, is the valve stem hole having a burr on the inside and wearing away the rubber around the stem of the inner tube, but that's super unlikely to be your case.
Try pic related if you really can't find the puncture(s) on your tubes consistently.
