I worked as a writing and math tutor for a few years. There are several possible reasons for this, and i cannot possibly diagsnose them, being not an expert and also having so little to work with.
But the most common cause of this is a wiring issue. Some people have a block of sorts between the creative and concrete parts of their brain. This means when being creative (writing, speaking about some things) you might struggle with words, especially complex ones. The way you would test if this kind of block exists is kinda complicated.
First step is to try and make up a story. Any story. See how well you do. speak it out loud. the try to tell another story, but this time, you must include these 5 words "grandma, tooth, sock, running, and blue". If you find you can create sentences just fine until you try to use one of these words, you may have the problem i am talking about.
Now try another test. Try alternating between telling a truth and telling a lie. Every other sentence. Try to make them go quickly as possible. if you find it difficult
There are a few other tests like these you can try and i can't possibly write them all in one post.
If this is the problem, there is good news, it can be overcome. You simply have to create the neural pathways between memory and creativity. how? by doing it. practicing these same exercises.
There are several other things that may cause these issues too, and if its really bad, i suggest you find a specialist who can help you. Though i realize money is a factor in many cases, and specialists aren't cheap. I only named the one that i ran into the most.
Disclaimer- these are tests the company i tutored for used, and i have no idea how grounded in actual science they were. All i know is the teaching strategies we used when someone tested positive for this did help to raise grades