How are you wired up? A standard PL2303 module will not have DTR broken out to act as a reset pin. Instead, they bring out the PL2303 chip's reset (input) pin which does not help.
Press reset on the promini when the IDE shows "Compiled xxx of 32xxx bytes".
If you select File:Preferences and turn on verbose outputs, can be easier to time the press correctly.
Some PL2303 modules have DTR broken out to a hole where you can add a pin or a wire so that you can connect it to GRN pin (green) and output a reset signal to the promini.