After closing the first warp gate where the vex pylons/crystals appear afterwards, that when shot down materialize a plattform to stand on and spawn the next ones. When I shot down the 2nd part with 2 of those, no plattform appeared and I had to proceed without doing it in heroic. Might be because I was trying to find out which weapon was most effective so I used a Sunshot first, but ran out of ammo before it was destroyed and switched to another. Otherwise I don't have any reasonable explanation.