I've had a couple instances where I was sure of my hit based on arrow fletch indications.
Case one: 535 gr arrow with an extremely sharp Zwickey Delta fixed 4-blade broadhead ran down the ribs of a decent whitetail buck. Shot was from a stand 10' up at 20 yards. Full pass through, arrow fletch said I hit mid-height in chest and expected a dead buck within 100 yds. With fresh snow on the ground and pheasant hunters in area that saw the buck I was able to piece together what really happened....arrow hit where I aimed but ran down the ribs and passed through creating only a flesh wound on the entry and exit below the entry. Fletch indications were good but the arrow had deflected toward the ground. Trailed the buck 5 miles before he bedded in snowberry bushes off the river bottom (measured by odometer as I was hunting a 13 mile stretch of public land along the South Platte River in CO). Blood trail started out decent but then really diminished as the trail went on. Two groups of pheasant hunters saw the buck and had no idea he'd been hit by an arrow as he was just motoring along. Bottom line, flesh wound on a "perfect hit" due to an arrow deflection on a rib. The buck undoubtably was chasing does again that evening.
Case 2: Same set up as case 1...sharp Zwickey 4-blade Delta fixed blade broadhead deflected on rib and angled forward in to the chest of a decent bull elk. Fletch indication was I hit the elk too far back. Shot was slightly down hill around timberline an hour after daylight. At the hit I was sure it was "guts", I waited 6 hours before picking up the blood trail....which was very good and led to a dead elk in 80 yards. The arrow had deflected off the last rib forward into the lungs, despite the fletch indications that told me that it hit even further back than actual.
After this event I made a "rack of ribs" that were placed in front of my foam target....small 1/4" diameter hardwood dowels pressed into a 2x4 on about 1" centers as I remember it. Shot the ribs at 20 yards with the Zwickey broadheads mentioned above. Arrows entered the foam target at anywhere from straight in from my position to 20* to 30* off-line from what was straight in. Convinced me right there that ribs can and do deflect arrows if the broadhead contacts the side of the rib.
Obviously every case is different, as you know and suspect. Hope you find your buck and tell us what really happened. May or may not be a mechanical not opening.