Yes I have experienced this. I encountered a Totodile I taged, walked 3 blocks to the Poke Stop, it never appeared, then it fled. I checked my nearby, it was still there. I tried several times to trigger it, but no luck. I trouble shot the best I could. I tried a Lucky Egg, Incense together, with the same results. I even took the risk of turning off my device, restarting it, that failed. So it is a bug in the game, it is full of them, but I love the game, will stick with it. Do not give up, keep trying.