I’ve installed the Lightbox plugin on a few site without a problem but when I added it recently to a client’s site it did nothing. No errors appear, but when clicking on an image it just opened that image by itself not in a pop-up lightbox.
I uninstalled and reinstalled. Nothing. Tried fancybox instead. Nothing.
Finally I opened the page with the image on it and viewed the source. With this pasted into my html editor I could tweak the page and by trial and error found the problem was a datepicker JavaScript loaded by the Custom Contact Form plugin. With this removed the lightbox plugin worked perfectly.
Fortunately the solution is simple: On the Custom Contact Form settings page, just change the option to only load the Javascript on forms pages to ‘Yes’. As long as you don’t want a lightbox image on the same page as a contact form then this will work fine.
Otherwise you can also (or instead of) disable the use of Jquery for Custom Contact Forms.