I recently discovered various Hydroponic missions on my low level character, so I decided to get some using my main character and my alt. But when i reached the galaxy, i docked at the correct base only to find that there was no hydroponics mission there on either of my characters, but it was there for my low level. I havn't done them before, and this same thing happened last universe. Is this a bug? Or is there a level cap on the missions?

More likely is you did them long time ago and sold the hydros as a low level. they are once only missions.