Some days scheduled scan very long time

Some days my scheduled AVG (for drives C for programmes and D for data):scan takes about 10 - 20 minutes, but some days, like today, it has been scanning for 2 hours and still only at 79%. When this happens, roughly every 10 days or so, have not noted the figure down yet, there are no threats to be found. I run Windows 7 64 bit Home Premium on a quad core modern 5 GHz machine. My e-mails are screened with MailWasher, and on other occasions, not when AVG scheduled scanning, I use SpyBot and MailWasher Pro.
This is a paid-for AVG Internet Security, version 16.151.8013
TuneUp version 16.74.2.606831
Framework version 1.143.2.51391
Setup version 1.143.2.51391 
Has anyone any idea why this happens and what to do when it is taking a very long time ? Would it help to cancel the scan and manually task one ? Why only now and then ?
I clear the Internet Explorer 11 browser cache/history but not passwords daily on shutdown.
Many thanks
Gerry

Hello Gerry,

We apologize for the inconvenience caused. The issue might have caused due to network failure when the program is updating. We request you to repair the AVG Internet Security program which is installed in your PC by following the instructions from the article given below and check if the scan runs fine. Here is the link: http://support.avg.com/SupportArticleView?urlname=How-to-repair-AVG

Thank you.

Hello Gerry,

We apologize for the inconvenience caused. The issue might have caused due to network failure when the program is updating. We request you to repair the AVG Internet Security program which is installed in your PC by following the instructions from the article given below and check if the scan runs fine. Here is the link: http://support.avg.com/SupportArticleView?urlname=How-to-repair-AVG

Thank you.