When a person is said to have failed to render, it usually means that they didn't provide help or assistance when it was needed. In other words, they were expected to do something (usually to help someone else) but didn't do it. This can be interpreted as a sign of laziness or selfishness, and it's often seen as a pretty big character flaw.
There are, of course, exceptions to this general rule. Sometimes people fail to render assistance because they're genuinely unable to do so (perhaps they're physically unable or don't have the necessary skills). And sometimes people don't realize that help is needed until it's too late. But in most cases, failing to render aid when it's clearly needed is going to reflect poorly on someone's character.