Quote (from Wikipedia )
"Green sand is not green in color, but "green" in the sense that it is used in a wet state. According to the Cast Metals Federation website, an alternative casting method is to heat-dry the molded sand before pouring the molten metal. This dry sand casting process results in a more rigid mold better suited to heavier castings"
That being said, the fact is that green sand is water-based and uses bentonite clay as a binder.
If the sand is too wet, it will be overly sticky.
If the sand is too dry, it will not adhere to itself.
Are you using a parting agent (such as talcum powder) to keep the sand from sticking to the pattern ?
Is there enough draft (clearance angle, no undercuts) on the pattern so that it can be removed easily from the sand?
Foundries use heavy-duty techniques to ram the sand into the flask. Are you compacting the sand enough?
One final bit of advice. Don't get discouraged. Green sand can be a pain to work with. Even the professional foundries can't get it right every time.
There are alternatives, such as Petrobond sand, which is oil-based.
It is much easier to use, has excellent adhesion, and can capture the finest details of a pattern.
I'm also a novice at this. I haven't made a perfect casting yet, but I'm still trying. And learning something new every time.