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Hello @Conservation
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your magnetic door stopper.
That is indeed a mystery why your magnetic door holder is not working properly when attached to the door. Can you please tell us the exact model number or share a picture of the magnetic door holder. Would you by chance have a steel framed door? Are there any other sources of magnetic force present in the immediate area of the door holder? Do you have a very heavy timber door? Sometimes it will appear that the magnet is not working but often a heavy door will easily pull away from a light duty magnetic door holder.
Once we see what kind of magnetic door holder you have, we'll be able to offer recommendations to get you moving forward.
Let me call on our experienced members @Dave-1 and @Nailbag for their recommendations.
If you need further assistance, please let us know.
Eric
Good Evening @Conservation
Want to know something odd? 
I was actually looking at the brassed coloured one Saturday I was impressed at the "snap" of the magnet to the post. (thinking of my front door)
With your question -
"Bought new door magnet to replace old one. Magnetic strength is good until screwed into door & floor, then no magnetic pull. If I unscrew one side it works fine. Any suggestions why this happens?"
I suspect it might be because the new door magnet is out of alignment compared to the old one. Did you replace both the door section and the floor section? The fire doors at work are very close to working/not working as in you have to open and hold the door for a sec so the vibrations are gone and then it looks solid. I wonder if your door stopper is just a little out of alignment?
Dave
You're not installing it wrong or there is a fault with the catch. In my experience with that style of magnetic door stop is that with heavy/solid doors they are pretty much completly ineffective in holding power. This gives the impression that there is not magnetic holding power. My own personal one below which is my rear garage door holder. Works fine unless there is a decent breeze and then it simply lets go defeating the purpose.
This clip in style or this one are as basic as it looks is far more effective without spending more serious money on a commercial solution.
Nailbag
Is there a magnet in the little piece that goes on the door? If there is, perhaps its falling out of place when you install it on the door. I suggest double checking to make sure that its properly attached before you install it again.
Eric
The photos below show the order of assembly. The magnet must be positioned centrally to the outer curved metal cover. It will magnetise to it. Then the spring. The door stop ball, needs to nest perfectly central to the curve of the door side magnet.
If assembled like this it will work.
Nailbag
