As discussed in two previous posts (here and here) Śālikanātha Miśra (IX c.?) dedicates several pages to the ontology of the ātman. These pages appear neither innovative nor particularly original, yet Śālikanātha must have deemed the topic important enough to include it in his Prakaraṇapañcikā. By contrast, Rāmānujācārya (after the XIII c.) hardly if ever uses the term ātman and does not discuss at all any ontological argument about it. That the topic of the subject interests him is, however, proved by the many pages about its hermeneutical and epistemological phenomenology.
Why does Śālikanātha feel the need to add ontological arguments, probably borrowed from the Nyāya school? And why does not Rāmānujācārya feel the same need? What happened between the two?
The following answer is more than tentative and it is based mostly on the method of investigating into the history of ideas (Ideengeschichte). It is easy to see that one of Śālikanātha's principal opponents, namely the Buddhist Pramāṇavādin, is altogether absent in Rāmānujācārya's work. In fact, Rāmānujācārya was active in South India and at a time (after the XIII c.) when Buddhism was no longer a philosophical concurrent. Consequently, Śālikanātha might have felt the need to defend the ātman also from an ontological point of view, in order to resist to the Buddhist anātmavāda, whereas Rāmānujācārya was active at a time in which the existence of an ātman was common sense among philosophers. Consequently, he could rather focus on its epistemological and hermeneutic phenomenology.
Vincent Eltschinger has attempted (in his book Penser l'Autorité des Ecritures) a similar solution of the different approaches to the Buddha's word in Dignāga and Dharmakīrti. Do you deem it legitimate to use extra-philosophical arguments to interpret a philosopher's thought?
On Śālikanātha on the ātman, see here and here. I dedicated several posts to V. Eltschinger's book. See for instance, here (first post on the book), here (discussing why we need Sacred Texts) and here (discussing an expert's different fields of expertise).