There is not luk elision of the case-affix after an avyayībhāva compound that ends in ā ; ām is the substitute of its case-affix es, but not when it is the 5th case-affix.,
This debars luk-elision which was to have taken place by the last sutra instead of luk-elision, we have आम् added to the words ending in short अ; for examples of this, see sutra 2.1.6; as उपकुम्भं तिष्ठति 'upakumbha is standing'; उपकुम्भं पश्य 'see the upakumbha.'
Why do we say 'after words ending in short अ'? Because after Avyayibhava compound ending in any other vowels there is not substitution, but there is total-elision of the case-affixes; as अधिस्त्री, अधिकुमारी.
But the ablative case-affix is not elided after Avyayibhava compound ending in short अ, nor is there the अम् substitution; as उपकुम्भादानय.,