AC doesn't cool room well in old poorly insulated apartment. What can I do to fix? What are the causes?
there is a proportion to be observed for the air to be cooled down, between the square feet of the whole area (home or apartment) against the basic "1 ton"
of equipment capacity.
One ton = 12,000 BTU which is barely enough force in your a/c unit to transform the air of 400 sq.ft from hot to fresh.

I hope this gives you the idea how to size the equipment properly according to the square footage.