I don't know how many times I've seen it. Two people duke it out on the line, and one wins. The loser walks over to me and says: "man, that guy just won't take a hit". Now, granted, in my years of fighting I have seen people who just cheat and disregard hits in an honor based game. However, I hear this sentence far more often than I see someone intentionally cheat. Sometimes, I even hear it from both of the fighters.
So what's actually going on here? Well, there are a number of possibilities. The most common is a simple matter of miscommunication. A shot was light or grazed off, so wasn't taken, but the person being hit didn't communicate why they weren't taking the hit. Armor, in particular, can make the situation worse because there is not a visual indication of them taking the first hit. It can be vital to communicate hit taking during a fight, or even after the fight was over, to prevent unnecessary interpersonal conflict.
Another possibility is that the shot didn't hit in the first place. Yes, you saw that shoulder wide open and delivered a good, solid hit and felt like it made contact. He doesn't say a word and keeps fighting. Cheating? Maybe not. It is possible you only made shield contact, even though it looked like shoulder. Since you didn't actually hit him, he didn't call anything.
So why did you think it hit him? The fact is, people don't see in 3-D, our brains estimate distance by triangulating where our eyes are looking. That combined with the fact that the angle between our line of sight and the sword's position can be drastic, causes us to misjudge where we actually hit. Remember, this is an honor based sport, so it is up to them to decide if they were hit. Don't assume your shots hit exactly where and how you think they did.
There is always a chance they were cheating. There may not be a reason to be upset with them about it though. A lot of cheating, especially from newer fighters, is unintentional. I fought for several years with a flawed understanding of where certain hit zones ended/started. Some new people just don't get trained on all the minor details like what the wrist bone counts as (arm, for those unsure) or exactly where on the shoulder counts as body (think sleeveless t-shirt). Other times, we see a newer fighter overwhelmed by a veteran to such a degree they either get confused or miss a subsequent hit while they were processing the first. Rapid processing of hits and displaying damage accordingly takes time to learn and train, don't expect everyone to be perfect at it. This, again, comes down to communication. Ask the person if your hit didn't land or what happened, you might have an opportunity to teach them a rule they didn't know.
I have seen, luckily on rare occasion, people who just don't play by the rules. If you have done your due diligence and given them the benefit of the doubt and talked to them, and they still won't take a hit, talk to a herald. Many people will tell you "just hit them harder" or some other advice, but this generally isn't going to work. If someone values their honor so little as to just not take blatant, forceful hits, hitting them harder is just asking for someone to get hurt. Sometimes just a herald being close by will solve the problem. If not, the herald will be able to take it up with the event coordinator or realm leader.
The takeaway I'd like everyone to get here, is that communication and trust goes a long way towards improving the quality of hit taking and fighting. Give your opponent the benefit of the doubt and talk to them if you aren't sure. Never be afraid to ask about how your shot landed or to explain odd situations after the fight.