Not able to delete some mail from the Smart Junk folder

Hi, I am having problems with not being able to delete some mail from the ‘Smart’ Junk folder. Most new junk mail shows up in there as unread and can be deleted without any problems, but occasionally I get one or more emails that show as already ‘read’ (i.e. not in bold) and they cannot be deleted whatever I do. Whether I press delete, shift-delete or use the right mouse button delete option, absolutely nothing happens. This is on a gmail account and the only way I can get rid of them is to log into gmail and delete them there. I only installed this a week ago and have the latest version. My other accounts work fine, it’s just my gmail acct. that does this. Any ideas?