I get that too, but I've come to believe these ideas in my head are undetailed, simply what you want/need to see in the image. Like a quick sketch on a napkin. You know what your image will represent, but details like perspective, tone, size of lines, even subject orientation (front view? side view?) are up to you as the artist.
But maybe that's just me.

Any ideas on how I can improve my memory? There is no such thing as a bad memory; only untrained and trained memories. I need to develop mine more.

One thing I came across: Draw something from reference. Draw it an hour later without reference.

The solution comes down to: DRAW MORE [=DRO MOAR!?!]

But STUDY the things you draw. That's why it's called a study ^^. You spend time to understand the structure, how the thing works and generally the knowledge sticks.

For instance take a mechanical crane/digging gizmo and observe how the pneumatics work, how is it able to move and basically, do what it does. Same goes for bone structure.

In time you will remember everything you drew/studied.

I guess thats the problem. Drawing more.

I got my ass kicked by a storyboarding class and it defined a style for me. Gonna draw more. Gonna study more. Gonna ADD less on shit.