So this topic could really relate to any role play game, but seeing as it happened in DH, i will start here.
Recently in a mission I was GMing, I introdcued a plot point, a data cogitator (Like a USB drive thingy) which contained information the party would later need. The problem is that the party forgot about it, which resulted in some issues.
The party in question was sent to a warzone planet, following a member inquisition they were working under for the time being. The inquisitor went on ahead, and the party was sent by ground to retrieve a data cogitator for her, and then bring it to her. The party got ambushed, and had to sneak into a captured facility to get it. On the cogitator was a list of imports and exports to and from an ecclesiarchy stronghold. Once they retrieved it, they handed it over to the inquisition, and were sent to investigate an archaeological dig sit being overseen by the ecclesiarchy and a group of archaeologists, all under the protection of the sisters of battle. Much talking, sneaking, bribing, and falling to their death later. They were looking for some stolen heretical books.
This is where my issue came up. They had followed all the clues (Far better and cleaner then I had expected them to), and had come to the end of the line. They had a good idea where the books had gone, how they had been stolen, and possibly why. All they needed was the information on the cogitator, which would point them at who did it. But the party had completely forgotten that the information even existed in the first place.
I tried hinting at it in game, even had one NPC describe it to them, as what they would need. Eventually I helped them figure it out out of game, but it was a while later. To the more experienced GMs, what are some ways to have small plot details that might not come into play right away be remembered? In the end we figured it out, but it was a bit of a frustrating buzz kill. Any help is greatly appreciated.