10 days is really too long to pitch fresh yeast. But it depends on why you think the yeast did not perform the way it should. If the yeast failed completely and the wort didn't ferment, then 10 days leaves the door open for botulism. Unlikely, but the consequences are pretty severe.
If the yeast stalled, leaving a lot of unfermented sugars or fermentation byproducts, then you can add more yeast to try to get it to finish but it's a bit more work than just preparing another starter. The yeast won't work well being pitched directly into an alcoholic environment, and may just cause the new pitch to stall as well. If you're fermenting in a bucket, you can lift it by the handle and give the bucket a few sharp twists to resuspend the yeast and maybe knock off a few more gravity points. Or, you can acclimate your yeast by adding some of the batch into your starter, letting it ferment, adding a bit more, and after a few steps add the active culture to the rest of the batch.
But as previously mentioned, take a gravity reading. At 10 days, it is probably full of diacetyl, acetaldehyde, suspended yeast, and all kinds of other fermentation byproducts. You need to give it enough time for the yeast to do its work, which involves cleaning up a lot of the intermediate compounds created by the yeast from the active fermentation.
If your gravity is still pretty high and fermentation stopped too soon, then you'll need a different fix.