Thestrals

If آپ recall, thestrals can only be seen سے طرف کی those who have seen death. Lucky for you, I was just compiling somenotes on them.
Harry doesn't remember anything of his parents' deaths except the light and the laugh. He was only a baby, after all. When we are babies, we have almost no capacity to hold memories. Try remembering what آپ were doing at that age. Impossible, isn't it?
Also, Harry watched Cedric die right in front of his face. Remember, he had nightmares about it for weeks, if we'er going سے طرف کی Dudley's insult.
It's simple psychology that kept Harry from seeing them sooner. If you'd like a مزید detailed explanation, I suggest asking your local psychology teacher.