Hi Theo,
Make sure that your site is actually loading jQuery. Check your dependencies and script enqueue priorities. When you enqueue it in the footer, you really need to know what you’re doing.
Keep in mind that many scripts depend on jQuery. It’s risky to put it in the footer, and we don’t usually recommend it because most people can’t pull it off without breaking some code. It’s one of those things that really only people who know they can pull it off should do it, at least on a production site.
I understand you’re probably going for performance improvements. Site speed optimization is one of the things we do day in and day out for clients, so we know a bit about this. If you do what you can to reduce code bloat, losslessly optimize your images, use CSS sprites, properly leverage browser caching, use a minification/concatenation plugin and a page caching plugin, and use other good optimization practices, jQuery won’t be your bottleneck…so you may want to consider putting it back in the header. We’ve got some sites loading in .6 seconds with jQuery in the header. Just my two cents on that.
Hope that helps!
Keep in mind that we said Compatibility Mode may help those who make mods and move jQuery to the footer. No guarantees. When you make mods like that, the debugging is up to you. ??
– Scott
