Thumb position pizzicato can be a fairly complicated subject. But briefly some thoughts.
Firstly poor left hand technique in all resisters will result in reduced sound. Varying, or reduced pressure, means the energy your putting in with your right hand the create the note is not all going to note production,but being dissipated where your not holding the string down enough.
In all registers using the weight of your arm is really important especially in thumb position. Rather than try to use solely your finger( and or thumb) to push the note, let the weight of your arm push down through your hand to hold the note. Also I find I need to move my right hand closer to the bridge to produce a stronger sound in thumb position.
I will write a blog post in the next few days on this, as there are a lot of other things to write about, and I will post it here.
Good Luck