The measure of good performance is good attenuation on the input reflection or in other words high Return Loss. Since it is loss a positive number is used. RL=15dB is generally considered minimum acceptable for simple circuits.
Impedance is geometric ratio of conductor width to gap to nearest grounds. Uniformity is key and matching impedances over the range of interest.
Details required on schematic, part specs and layout to understand your issue. Photo's schematic etc.
Insertion Loss and Return Loss can both be affected by common changes, but understanding how each part contributes and how to test it , is a learned skill.