Carry a good pair of binoculars, 6 to 8 power.
Like 8 x 21mm, 7 x 35mm etc.
They don't have to cost a lot.
The one I use is a old pair, Sunscope 6 x 30.
When you get within shooting distance the squirrels, will hide in tree forks, or just hug to the tree trunk, and blend in to the bark.
I took my binoculars just to try them out, and found out why I wasn't seeing any, they were there I just didn't see them.
I was surprised at how many squirrels I had missed because I couldn't find them.
When I see or hear a squirrel, I only sit about 10, to 15 minutes, looking up and down the trees.
Then I move on.
Try a set of binoculars, your squirrel rate will go up.

Good Luck