The kit says I can use up to 30mg of tissue. When I used 15mg I got a higher yield than when I used 30mg. Why?

For your particular sample, 30 mg may be overloading the column. The 30 mg recommended is the maximum that can be used with the kit; however, this may vary between tissue and species type. The ability to recover DNA or RNA depends on efficient lysis and the binding conditions set up during the pre-column steps.  Too much material can alter the binding conditions, reducing yield.

Large amounts of contaminants can also reduce the binding capacity of the membrane. Some tissues are particularly rich in DNA, specific proteins or metabolites that can have a negative effect if present in large amounts.