gd day, recently FCd my druid from ally:Alliance: to horde:Horde: and realised that i couldnt get that specific quest "again". although im eligible to accept the two other quests inside WC. (i ve done the 3 quests on the alliance side). Might be worth to mention i was able to complete the chain quest that includes in SFK on both sides. So was basically wondering if it would be a matter of bug or if its simply normal, for some reason (?).
Quests available to both factions can not be repeated after a faction change. If there are separate quests for each faction then they can. If in doubt, check a few sources like wowhead (you could check thottbot, but regardless of what the comment is about it probably just has something like, 'lolhunterloot' written).
Example A:
Back to the Orphanage(H) and Back to the Orphanage(A) - you can complete one version, then faction transfer and complete the other version because they are actually separate quests, even if they are for all purposes identical.
Example B:
Cleansing the Caverns - there is only 1 version of the quest, which is listed on most sources as 'Both' with regard to faction. There is only 1 version of the quest, so you can only ever complete it once.
Generally if a quest starts and finishes at a different point for each faction then it can be repeated after a faction change, even if the name is the same.