As per a source in an intelligence agency who has direct access to DG NAB Peshawar, the delay in the issuance of appointment letters is due to a stay order from Islamabad High Court sought by current NAB employees on account of their promotions. This is 100% authentic news. This news is also confirmed by a director of NAB who is stationed in NAB HQ Islamabad. The guys who pointed out the stay order as being the reason for delay in the recruitment process are absolutely right.
Personally, I have little idea of the detail about the nature and timings of the stay order (surely, you guys can't expect me, being a humble creature, to grill an intelligence official and a director in NAB too long about questions related to details of the stay order).