Delay in period after Depo provera.

My wife had her last period ended on 28 nov'17 and she got her first depo injection on 2 Dec'17. After two months of period delay we visited to a gaynae and she prescribed some medicines for period one was livogen and two were more. (Note: my wife forget to mention to gynae that she taken depo injection). So after taking full course of medicines for period return we visited again to gynaecologist and this time we mentioned that she taken depo injection. My gynae said then in this case we have to wait for 3 months and period will return on 4 month. Now 4 months completed but still my wife period is missing. We took self pregnancy test many times but all were negative. So how long we have to wait? Is this normal not to have period after 4 months of depo provera injection?