What are your human playback (HP) settings?

By synth patches, do you mean Aria Player (Garritan) or another software synth?

The staff that you have the synth sounds on, does that staff have a name assigned to it?

The synth that you're using, what controller controls volume on it, CC#1 (modulation), CC#11 (expression), velocity or something else?

J Adrian Verkouteren:

I’ve been using “Custom” but it makes NO difference whether I use Custom, None, 21st Century, or any other. 

I mean the “Synth Pad” selection option offered in the score manager.

Jeffery Turner:

Yes, “Synth Pad”

The Mixer is the ONLY thing that will control the volume level playback.

On my machine Synth Pad responds to set dynamic markings but not to haripins.

The change is more pronounced when played through Finale's MIDI sounds than when played through VST.

So I think I can see the situation you are facing.

(Using the VST "warm pad" did respond to the cresc. hairpin.)

So Adrian, is that just another "glitch" in Finale, or is there something I can do to control the volume on my synth sounds?

So Jeffery,

my answer to your questions are as follows: 

Yes, “Synth Pad”

The Mixer is the ONLY thing that will control the volume level playback.

The Synth Pad responds to volume indication such as pp, mf, ffff.

So it does respond to volume changes, just not continuous changes such as hairpins.

Whether this is a glitch or not depends whether the MIDI synth pad was designed to respond only to velocity (like a piano) or to continuous controllers (like a clarinet). You'll have to ask the folks at MakeMusic whether the current performance is a glitch.