Howdy! This is my first post on this forum so bear with me

When I try to make a dimension it simply doesn't work. I have been using solidworks 2016 for around a year now and this is the first time I have ever had this sort of problem. I recently reinstalled it after reseting my computer and it was fine for a few days and then, seemingly out of nowhere, this happened. I have recorded a video and posted it unlisted on youtube. here is the link: Solidworks Smart Dimensions not working - YouTube

Thanks in advance,

Alan.