Calling all squirrels......

[Mr. 16 gauge]

Went out on Sunday to do a little squirrel hunting. To make a long story short, didn't see much of anything....one black phase grey put in an appearence for a few seconds, then disappeared (before I could put the cross hairs on him!).
I have a question re: squirrel calls. I bought one on a whim a few years back, and the first time I used the silly thing I got a reply from three different squirrels at the same time, two of which came in! on a dead run!

I have since used this thing (its a bellows type that you tap the end of the bellows and it chirps and squeaks) without any success. Is there any special way or technique to use a squirrel call? To be fair, the conditions on the weekend weren't all that great.....a little windy with rain off and on.

Any help you could give me on how to specifically use this thing would be great......I thought I was in a good spot for squirrels...there were acorn cuttings all over the place!
Thanks in advance..........