The flash rate is determined by the amount of current drawn by the circuit. It heats the bi-metal strip inside the unit which makes/breaks the connection.
Normally, when the flashers are slow (or don't flash at all) it is because there is too little current - ie the battery voltage is low or there is a poor connection(s) (or one of the bulbs has blown!)
Too fast means the opposite.
Wrong bulbs? - should be 12V 21W
Battery voltage too high
- measure at 3000rpm and let us know (should be ~14.5V)
Of course, the unit could be faulty. Is it the correct one?
When you see cars or vans on the road with this problem, it is usually because they have a trailer unit fitted and it's been wired up wrong!