There is a fixed list of rumours, where they are, what they say etc etc. Just because you've read one, I don't think it means you can never read it again. It might do so until you launch, but I've definitely encountered the same rumours more than once while I've been playing

so how come whenever i travel to planet crete and other bases rumours wont appear? they always show up as speech bubbles without writing and the ppl always say...sorry nothings going on

There are chances that you are able to read a rumor that you already read before. But those rumors generally give information about specific weapons, where to get ships and which shields are the best to take on a certain faction.

Rumors that are focused on the base itself. Like on Crete, all the rumors that go about the history of the Corsairs, the initiation test of the new rookies, etc. are, at least to my knowledge, only showing once. But I could be wrong of course. I am not an all-knowing oracle.

So if you have read all the rumors, you will see the "empty text icon" every time they have no mission or bribe for you. You could however, get the rumors of that base appear again. But to do that, you need an ini editor (like biniqdu), an infocard viewer (Like fled-ids) and a dll editor (like reshacker)

Check the downloads section of TLR. You should be able to download all of them.
(I think)

It will take quite some time to write everything down you need to do and I don't have the time at the moment, but if you really want me to explain it to you, just tell me, and I'll write the tutorial when I have the time.

It's also possible that someone has a far shorter way of getting the rumors to appear once more.

Unfortunately, the rumors only appear once. Where the rumors are and when they will appear are in the mbases.ini file, as well as in each player's saved game file. Each NPC on a base has a group of rumors that they will give to the players. If you look in the mbases.ini file, you will see many enteries like this:

[GF_NPC
nickname = rh0202_rnavy_001_f
. . . . . removed
rumor = base_0_rank, mission_end, 1, 133922
rumor = base_0_rank, mission_end, 2, 133923
rumor = base_0_rank, mission_end, 3, 133924
rumor = base_0_rank, mission_end, 1, 133925
rumor = base_0_rank, mission_end, 2, 133926
rumor = base_0_rank, mission_end, 3, 133927
rumor = base_0_rank, mission_end, 1, 133928
rumor = base_0_rank, mission_end, 2, 133929
rumor = base_0_rank, mission_end, 2, 133930

The first, second, and 3rd items are the conditions that must be met for the rumor to be given. "base_0_rank" and "mission_end" mean basically that it is either after everything has been completed in Single Player, or else anytime in Multiplayer. The numbers "1" or "2" or "3" are how many times the player must talk to the NPC before they will give the rumor. So you can see that some NPC's will only give certain rumors after you have talked to them 3 times. Once a player hears the rumor, they will never hear it again. The last item "133924" is the infocard location where the actual text of the rumor is stored.

Now, in your player saved game file, there are several spots that store the rumors that you have heard and the NPC's that you have talked to. For example, there is the line:

vnpc = 3094291273, 2590178767, 1, 0 ; Bw07_01Base, 0x9a62fdcf

Basically meaning that you have talked to that NPC 1 time. Also there is the line:

rumor = 253711, 1

Means that you have already heard that rumor, so you won't get it again.

So, to get more rumors, you either need to add more to each NPC in the mbases.ini file or even add custom rumors in the infocards, or else edit your char (of course only in Single Player) to remove the lines for "rumor =" so that you'll see those rumors again.