I've seen some people think that during that scene Cap's eyes turns yellow or orange and either he is powered up or that is not Cap but someone imitating him.Ultron should have been able to swat him like a bug too but if he's able to block a shot from Thanos like that without having had some kind of major power up then this is even more implausible. Only other possibility I can think of is Thanos is just fucking around with him for shits and then swats him like the bug he is.