Blank spaces in Portfolio

Hello again. I have an issue with my Portfolio feature. When I increase the number of items from 3 to 6, two blank spaces appear, which in desktop mode prevents the items from being in a nice, attractive 6-item tile. I do have Jetpack, which might be interfering with it somehow. I have turned off and on the "Portfolios" feature in Jetpack, and it doesn't seem to make any difference. The only thing I changed at one point was the dates of the posting/publication of the portfolio items, pre-dating a couple items in an attempt to re-order them in the way I wanted them to. How that led to creating blank spaces in the 6-item portfolio is confusing, but maybe you all have a solution? Thanks in advance.