The reason may be that you have the wrong American English placement.
Wait...what is placemen?
I used to have no clue; in fact, I had never even heard about placement until I had already been teaching for years! Yet, what I've found about placement is that it is one of the biggest factors to achieving a natural accent and pronunciation system in any language.
Placement refers to where you project sound from when speaking.
Now, be careful with this! Many students feel their vocal cords vibrating and think it means their placement comes from their throat.
That's not entirely true! Your vocal cords are located in your throat, so you have to feel some vibration there. The reality is that even if your vocal cords are vibrating, your placement may be coming from elsewhere.
For many languages, the placement tends to be coming from somewhere in the mouth, like towards the back for Mandarin and towards the nose for Portuguese.
For American English, you want your placement to come from low in the throat. To get an idea of the difference this can make when your placement is in different locations, make the /i/ sound in your first language (think of the word "sleep"; that vowel sound is /i/).
Then, ask a native American English speaker to say the word "sleep".
Compare how your pronounced the /i/, and how the native speaker pronounced the same word.
The difference may be slightly due to mouth position, but the larger factor will be where the sound is being projected from.
Obviously, this is a difficult concept to convey in writing. So, I encourage you to check out this video from our Fluent American channel to start getting to work on your American English placement, and hearing examples from different languages: