Seeking zeroing advice - Red dot sight is always far too high (even with max adjustment)

Hi, I recently acquired this Ruger AR-556 (pictured) and I am seeking advice to help me zero my red dot sight. This is my first rifle.

I have: - swapped out the stock & grip - added Holosun HS510C - added Magpul bipod

I haven't taken it apart, I believe it is brand new.

I took it to the range for the first time recently to zero it - it sounds like a straightforward procedure.

My issue though is that my shots are always coming in much lower than where the red dot it. I have been testing it at 25yards, put probably 100 rounds through it.

I've adjusted the red dot to the lowest point it will go to, but it is still far too high.

Can anyone help me understand why this is happening and what I can do to resolve it? Perhaps I should be using a different optic or have it shimmed? Any advice is appreciated. Thanks!