There may be many reasons, and one reason may be yourself. Maybe it is yourself that stops you from being creative.
Days when you find it hard to be creative, grab a paper and a pencil and put down the words that explain why the creativity does not work today. Make a list of at least ten, maybe 20, words and/or phrases.
I guess that most of the words and phrases ore more or less negative. Consider why you chose these words.
Maybe you can give other names to the problems? Maybe these negative phrases also can be explained as opportunities? Maybe they are the challenges you need to enhance your own creativity?
Go back to your list again and this time you should formulate each word/phrase to express a positive option or possibility.
Feel how your creativity starts to bubble inside you. And in the future – only use these positive words and phrases.