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What tablet do you have?
Is the tablet running the stock UI? You haven't installed something like Nova Launcher or anything on this device, correct?
Do you have any "saver" apps running on this device? Memory saver, Data saver, Battery saver, Anti-virus, Malware or similar apps.
Have you ever gotten these apps to close from the multi-tasking screen? Have you tried tapping Close All Apps more than once? If it looks like all of the apps have closed, you leave the page, return and find a refreshed app list, have you tapped Close All Apps again?
Is it always the same apps not closing? How many apps are there that do not close? If not always the same apps, is it always the same number of apps not closing?
Please attach a screenshot of the multi-tasking screen before closing all, after closing all, after closing and reopening from an empty list, and any others that help show/explain what you're seeing. Please redact any personal or identifying information from your images.
Have you tried force closing any of these apps from their app page in settings? To do this navigate to Settings>Applications>(the apps you feel are not closing)>Force Stop
Are these apps, that you feel are not closing, pre-installed apps?
Has the behavior of this table changed or has it always been this way? Was there a time when you able to close everything with one tap and now it has stopped working? If the behavior has changed, about how long ago would you say it was since it started?
While I could imagine how an email app might keep reopening itself, not that I believe it is, but email is at least important. The stick in that is the other apps. I mean the QR app has no importance.
Troubleshooting
The first thing I would want to do would be to figure out exactly how many and which apps are not closing. To figure this out open some random apps, maybe 8 to 10 of them then go in and look at the list. See if all of the apps are displayed on the same multi-tasking page. I've seen some devices split the multi-tasking list after so many apps, older apps are pushed to a second or even third page, where the system only displays 8-10 of your most recent apps and everything over that is actually on another page. This is when I personally have tapped the Clear All Apps button only to reopen the multi-tasking page and finding it repopulated with apps that weren't in the original list. This is why I asked if it's always the same apps and the same number of apps. If you open 12 apps and find 2 that don't close, did you see those two apps in the list when you pressed the clear all button? If you open 15 does the number change to 5 apps that didn't close? Do you notice if the apps that remain are the first apps opened, or it doesn't matter it's always the same ones?
Let us know what you find for better advice.