Prestashop can get stuck sometimes, and fail re-compiling cache files with your newest modification. If you come across this issue, click on Advanced Parameters -> Performance, and hit the Clear Cache button on the top right corner.
If you are also having issues with PHP being cached, because you are using APC or another caching system, you can also try out the free Prestashop Cache Clearer Module developed by Nemo. To use it, simply install it from the module's page and run the standard Clear Cache.
