B1-B2 Visa administrative processing

Hi, I have applied for B1/B2 visa in 04/10/2017. VO told my visa is approved during interview but when i went to collect visa and passport on given date ( after 4 days) my passport returned without visa and any slip etc , l was informed consulate will call me when ready. No slips given or additional documents requested. CEAC website says administrative processing. Is this normal. I am Indian resident in Oman. What could be the reason for administrative processing perticularly without asking for any additional document.

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