I've been frustrated by this problem as well (most commonly with the Ixal quests, but I've seen it come up elsewhere as well). Whenever an NPC has exactly one quest to be started or progressed, but other active quests with dialog options, you cannot reach that dialog for the other quests.

In some cases, that dialog-that's-not-progression is even necessary to continue a quest (like supplying replacement materials or refreshing a buff so you can make another attempt at an Ixal craft). In other cases, that extra dialog provides the story of the quest. (One whole tier of the Ixal dailies sent you to Fyrbryda to collect items she needs in exchange for letting you use her crafting facilities. It's only if you talk to her again afterwards that she goes into an explanation of why she needed those items and how they helped her and others.)

It's frustrating to not be able to get to that dialog without taking another quest first. It can even be frustrating just being locked out of small talk if an NPC has a quest we don't want to take (or advance) yet.

I'd really like that selection dialog to always come up before a quest window or quest dialog. If what I want is to take the next quest, then I'll select it. But give me the option to choose other dialog as well whenever there is any.