Filed 2/14, Stuck in Processing: IRS Submission Center Austin Letter Requesting Income/HOH Verification Docs, 3/27 Deposit Date Missed

Filed on February 14th and still stuck in processing. I got a letter from the Internal Revenue Service Submission Processing Center in Austin, TX 73301 asking for verification docs. The letter was addressed to me (had my name on it) with "Official Business Penalty for Private Use, $300" marked on the envelope. I already sent everything they asked for - my paystubs, proof of my kid living with me, and divorce papers to claim head of household status. I'm getting really worried since my 3/27 deposit date came and went with no refund in sight. No transcripts available online yet when I check WMR or the IRS website. The letter had a reference number that started with "Ston 4106 AUSC" which I assume is some kind of IRS tracking code. I've tried calling the IRS but can't get through to a human after being on hold forever. Anyone else dealing with this kind of verification delay? How long did it take after sending in your documents to finally get your refund processed? I'm starting to get worried that something is wrong with my return.