Image to Youtube (only 10 links work?)

Hi, i’ve been working on https://www.dansdrumlessons.co.uk where i have 150 drums lessons in a course.

The 1st page i built had 20 lessons but only 1-10 of the links worked, i thought maybe each page has a max linkage of 10 so i’ve now put max 10 lessons per page but after trying to link the 2nd page, lessons 11-20 none of them work.

I’ve even unlinked lesson 10 to see if i can get lesson 11 image link to work but it doesn’t – i’ve re-linked 10 and its working fine.

I havn’t got a clue what to do?

Thanks in advance
Dan