Moralities do not metamorphose as they progress on some imagined cycle; Slave morality does not change into Master morality: rather the two moralities are distinct polarities who are in a constant struggle.There is no single Master/Slave/Master/Slave (ad.infin.) continuum.
Masters do not 'morph' into Slaves, nor vice versa; the dominance of Slave Morality means that Master Morality has gone underground, or has been EXILED [self-exile].

The terms 'Morality Cycle', and 'Slave Cycle' do not appear in Nietzsche's writings.

There is no single moral continuum - there are many 'moralities' just as there are many perspectives.The 'Eternal Recurrence of the Same' works with the diverse complexity of all organic and INORGANIC becoming;'Morality' is a human interpretation of phenomena, not a thing in itself.Ultimately it describes the expression of Will to Power, and relates to the main polarities of Strong Will and Weak Will.

The fact that Nietzsche identifies Master/Noble means that the term is QUALITATIVE. The Master race is Noble, a master over itself, the enviroment, the animal kingdom, and subject races. If it loses its power of subject races, and the subject races go on to parasitise the 'seat of power' it does not make them Noble; indeed as history shows us, when subject races are given what in modern parlance is called 'self-determination', they show themselves unable to achieve high culture let alone stable governance. In other words they remain Slaves [in the qualitative sense] even though they may bedeck themselves in what seems like a parody of their erstwhile Master's livery.

Why is this? Why is the Slave Rebellion doomed to fail in qualitative terms - why doesn't it produce a Homeric Greece, an Augustan Rome? - why isn't the assumption of political power itself enough to bestow Nobility?

Simply because behind this moral interpretation looms the basic polarity of Strong Will/Weak Will. So the Master/Noble equals Strong Will, while the Slave/Revolutionary equals Weak Will.

Weak Will is characterised by the Gregarious Instinct; everything gravitates towards association, communality, egalitarianism. This is the Herd Instinct; 'strong' in numbers, but NOT in Will. Strong Will always wants to go apart, it always wants solitude, it always wants to distinguish itself; it always wants to elevate itself , to overcome itself and the Herd.This latter is very rare, but requires strong Will in the qualitative sense - it is Nobility per se.It is not the debased pose of the 'rebel', it is rather the Evil, Wicked tendencies of unique, sovereign, immoral, overflowing individuals.

So, in the Eternal polarity of Strong Will / Weak Will, there will always be the few who seek to go alone/apart; when such form a group of like minds, a Black Order, they will seek to subjugate man and nature, and therefore become a Master race.However, there are Noble types who rule over nothing but themselves; they may be in exile, they may be biding their time before making outward conquests; likewise there are Herds who are without masters [have never had masters] and exist at the lowest level of culture; they may subsist at this level for aons; they will have a basic morality which will say the Herd is all, all are equal in the Herd.

Nietzsche describes what happens when a Master/Noble Race extends its power and begins to subjugate human Herds; he states in Genealogy of Morals that this occurs like 'lightning' [GM II 17]

As long as Strong Will exists, Master/Nobles will always recur, just as Slaves will always seek to congregate: they are both at an eternal Distance - they never 'become' each other. To quote Kipling;'And never the twain shall meet'.