Not sure if this has been said or not, but flasher operates on it's own circuit and doesn't share the same lines as dipped/main beam. It does share the same relay though. Ultimately, this means you can "flash" without having side lights or dipped lights on.
I will assume you've checked all fuses, maybe try changing your dim-dip relay, if it's old the "flash" terminals might be burnt out. If this doesn't fix, and your main beam functions when you push stalk away from yourself with dipped lights on......then it's a problem with the live either before or after the stalk, multimeter should confirm, alternatively use a length of wire to hotwire from the battery.
Alm.