Gaps on Product page!

I have been adding new products to sell and have noticed that they are being displayed incorrectly.
https://www.silverbirchjewellery.com/?product_cat=pendants

I have three images on each row but they are now randomly leaving a gap instead of having three on a row some only have one. On the page example above I do only have the 11 products you can see for the ‘pendant’ category so its not as though it is missing the product it is actually leaving a gap where the next product should be.

Any ideas anyone?