"Nation" is really a misleading concept to describe anything about Andalus (or the Islamic world in general, really, in most cases).
They were entrenched and established IOTL something like a couple of centuries before the "French" or the "Spanish" were anything worth talking about, let alone united, but that still did not do Andalus any good. The issues were manifold, and largely about a messy internal structure at several levels. Ethnic divisions (mainly between Arabs and Berbers, but also within both groups) played an important part.
The power structure at large was plagued by chronic instability, with a layer of questionable legitimacy.
This was, by the way, largely the case of Medieval power systems in general, exacerbated in al-Andalus by the aforementioned ethnic fault lines, a major rift between the military power and the civil society, and a unusually militant set of neighbors both on the Northern (Latin/Christian) and Southern (Berber/Maghribi/Muslim) sides, among other issues. Rivalries among local areas/cities also played a part I think.