That's because when a Dreadnaught has already been defeated by at least one group of pirates, the sigil will disappear after some time.
So if you're literally always in the Dreadnaught, chances are other players had went in and already took it down. You'll have to shoot up the Dreadnaught again and board it just in time if you want to get a chance at doing the dungeon again.
There is a glitch with the area for the haywire armada enemies. It's like when we take down the the battle station we can't go in at all. no sigil or anything. please fix this.
This has happened to me a few times and I know how you feel, it's SUPER annoying! I mean you wait for who knows how long just to board the Dread, only to find out that the sigil's not there. I know people have said that the reason is a group has already completed the fight, the thing is, I had once docked at the dread, right after defeating it in the skyway, and yet the sigil still wasn't there. I know that a good group of all maxes can finish the fight pretty quickly, but the transition from skyway to dreadnaught was so quick I couldn't imagine anyone finishing it that quickly.