Can't get all the stain off

You probably should sand it off instead.

My deck is about 25 years old; pressure treated wood. I just had it power-washed. The guy who did that job offered to give me a quote on restaining the deck. He said he would lightly sand it. He explained the pressure washing would not remove all of the layers. he said at least one layer was incorrectly applied and he couldn't get it off with pressure washing.  he said nothing about chemically stripping it. I certainly know there is a right way and a wrong way to do any project. I'm not physically able to do the task myself and would just like everything to be a uniform color for the next year or so. I would really like to have a screened in porch instead of the deck so that is the reason why I don't want to spend a lot of time and money on getting this back to perfection. If I go forward with the porch project the old deck would be removed. a photo is attached.

You have a solid stain so it cannot be stripped. Your options are to sand it off to go with a semi-transparent or cover it with another solid color stain.

Ok, thanks. Is it recommended to sand the surface for better adherence of the new coat. Would a pole sander do an adequate job or is this one that needs to be done on hands and knees? what about an OSHA mask? And what coarseness of sandpaper should be used

Pole sanding will not work. You will need a power sander of some sort. You do not need to sand if suing another solid stain. Just pressure wash off the dirt and any loose stain.

That's great news. I just had the house and deck power-washed and I plan to use a solid stain. Looks like I'm good to go. Thanks so much for the quick response.