App store asks me to sign in repeatedly when updating a specific app and won't update it?

My app store on my MacBook Pro will not allow me to update the November 2016 update of iMovie. Every time I try to do so, it just keeps asking me to sign in. I of course do so, but then it just keeps on asking and asking and it never allows me to update it. I don't particularly use iMovie, so this is not a direct issue. However, it's frustrating that the app store will not allow me to update it when it allows me to update everything else! i have tried signing in and out of the app store multiple times to no avail. Has any one else experienced this or can advise on the matter?