I have a mulching mower ,it always leave a strip of uncut grass when I mow, took mower to the shop thinking it was not running fast enough, that wasn't it do yiou have any ideas ?

You might need your blade sharpened or replaced.

How does this strip line up with the mower wheels, is it between the wheels or right under a wheel?

Mulching blades are different. But it would be nice to know the pattern and the grass height at the time of the mow.

Sharpening a mulching blade means sharpening the edges where the blade is bent. Sometimes troublesome to do.

Just sharpened the balanced the blade , that was the first thing I tried . As for where the wheels run, that's not where the strip is, it is right in the middle of the path, a little to the left maybe more than the right, we mow once a week, some times the grass is a bit high

Two bladed or just one? Any chance you installed the blade upside down? A mulcher is supposed to suck gass upward where is swirls around getting cut over and over. An upside down blade would blow grass down toward the ground so the blade would miss it.

Single blade ,and it is on right

Hi,
Mulching decks should be level or one notch lower at the front (counteracts the fact that it moves in a forward direction), should never be cutting more than 1/3 of the blade of grass and be moved at a speed that produces the best cut. That is almost a direct quote form a manual for one of the first mulching mowers mad and is over 25 years old.
Best of my knowledge, no one has ever beat the Jacobsen two-blade mulcher for results. I past customer of mine was distraught when his doctor would no longer permit him to mow because of his heart. He used one and had not ever fertilized his yard; the best yard in an upscale neighborhood in the 3rd wealthy community in NC at the time (20 years ago.)
Peace,
Clarke