Most comments fail for the same reason: they could have been pasted under any video. "Great video!" tells the creator nothing, gives viewers nothing to answer, and looks exactly like the generic praise that spam filters are trained to ignore.
The fix starts with a detail only a watcher would know. Name a moment, a timestamp, a number, or a claim from the video. "The 12-minute example finally made nested loops click" proves you watched in a way no template can fake.
Then ask one genuine question the creator can answer in a sentence. Questions about a choice the creator made — why this ingredient, why that camera setting, why this bridge in a minor key — are easy to answer and hard to ignore. A question nobody can answer is just decoration.
Say something only someone who watched could say, and mean it. If your comment works equally well on a cooking video and a coding tutorial, rewrite it. If it only makes sense under this video, you are close.
Leave self-promotion out entirely. A link to your own channel is the fastest way to look like spam, and it is the pattern YouTube's own guidance uses as its example of what not to do. If the comment is good, curious viewers find your channel on their own.
Before you post, score the draft. Tarelivo's free score checker grades specificity, relevance, value, tone, and length, and flags links, self-promotion, and sub4sub-style phrases. Fix what it flags, re-score, and only then post — in your own voice, from your own account.