[Q] how can I sort out apk installation problem?

Hi guys.
I don't know if anyone else has had this issue with downloading apk files from unknown sources.
OK so whenever I download any apk from sources other than the playstore. I know I should check the 'download from unknown sources' in the security section in the settings menu.
However when I press onto the apk and the installation manager asks to either 'install' or 'cancel' I try to press install but it doesn't work... Like no reaction on the screen... It's as if the screen is locked. But when I press the cancel button it works N hence apk isn't installed....

Does anyone have any solutions to this please let me know.
Thanks

I have attached a screenshot of which button doesn't work.

Your help would be much appreciated

speedyjay said:

Get rid of that task killer for starters! You dont need it, you're just killing apps that are gonna start up again so you're going round in circles. Unused RAM is wasted RAM.

Sent from my SM-G900F using XDA Premium 4 mobile app

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Thanks mate it does make sense, I shall be deleting that.
And regarding the problem, after trying just about everything I wondered if my lock screen widget (echo) could have anything to do with it. So I deleted it, and presto! Install button is working!

I guess it was the widget app somehow messing with the touchscreen function on the phone.

For anyone having problems similar to what I had get rid of any lock screen widgets, screen dimmers etc.

Tc.