Sounds like your vacuum pump is capable of pulling a vacuum.
The first thing you might want to check is to see if the vacuum pump
is pulling a vacuum to the table. Put your finger over the vacuum
hole on the casting table. You should get a solid vacuum reading.
There could be a clog in the line between the vacuum pump and the
casting table or the valve that directs the vacuum to the casting
table might not be working.
My guess is the investment is not flush with the flask edge.
A vacuum on the flask can not be pulled if there is any investment
below the flask edge. The investment below the edge of the flask
allows the vacuum to leak out.
Before I put my flasks in the burnout oven I use the edge of a
trowel to scrape the investment flush with the flask edge.
The trowel I use is curved so the investment is slightly concave. I
think the concaved surface in the investment provides better
distributes the vacuum to the flask.
Even with that I will every now and then have a flask on which I
cannot pull a vacuum. I take the flask and rub the bottom edge on the
rough stepping stones I have as a floor in my casting area. That
always fixed the problem.
My paper blog shows my casting procedure. A ways into the paper you
will find drawings of what I mentioned above.
If you have difficulty finding the sketches let me know and I will
send sketched off line.
I did have a warped vacuum table once. I was using the table
regularly until one day I could not get a vacuum on the flask. I
tried all sorts of fixes but none worked. I finally checked out the
flatness of the table and found it warped. Luckily I had a backup
table and casting continued without interruption.
To this day I don’t have any idea what warped the table.