Issue with turn signals in 70 Dart

I'll bet it's the bulbs are not properly grounded or defective.
I've even seen new ones that don't work out of the box new.
Maybe try some known bulbs out of one of your other cars and see what happens.
If not, i would try the known flashers out of the same car to narrow it down.
Then if that doesn't work, start tracing wires in the whole system.
It honestly sound like it's a bad bulb or bulbs to me........