Gallery Thumbnail Alignment Issue

Hi guys,
My homepage consists of various galleries, each being 1 large image with a 5 column wide set of thumbs below.

I updated to the latest version and added some new pictures yesterday and I now now experiencing a strange issue.

There are blank spaces within the thumbnail sets. So for 1 particular gallery there may be 2 full sets of thumbs 5 columns wide, but the next row the 1st 2 columns will be blank, but columns 3 4 and 5 will have thumbs as usual. then below the 1st 2 rows will be populated.

Not all galleries are affected but the ones that are look very strange! I hope you can help.

Thanks in advance,

Louis