You may need a different flasher can that can carry the load of all lamps.
A simple starting point check for this is to remove the flasher can you have and then short the wires in the loom where the can was, together.
All the Indicators should come on, but not flash when the Hazard switch is pressed.
This will show the relays are correct and it's wired right.
If that works, then look up a flasher can that is suited to 4 x 21 Watts (most are only 2 x 21 Watts). Many of those suited for Trailers would be OK.
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