Well... this isn't the best analogy, but... imagine a balloon (uterus). It's been blown up and the opening to the balloon tied shut (the cervix). Now, inside the balloon is a rubber ball (baby head). You can hold the balloon by the knot and move it around. And you can watch while the ball rolls to the lowest part of the balloon.
So assuming the opening to the balloon is the cervix, and the ball is the baby's head, what could be happening here is that the opening is not the lowest part of the balloon. The balloon is still at a bit of an angle. You can see and feel the ball sitting there at the bottom of the balloon, but the opening is still angled up a bit... so you see the ball but not the opening.
The birth process pulls the opening of the balloon down towards the "floor" while untieing the knot and eventually the ball slides down and out from the now untied and downward pointing opening.
Again apologies for the totally silly analogy, but it's what came to mind!

And it's the same thing that's happening with me... each midwife has had to really push to find the cervix, even though babe's head is engaged and very very obvious. I think it actually is more common to happen after you've had a kiddo since the abs tend to relax a bit and let the uterus tip forward a bit more.