There are a number of reasons why you might be running into this issue - most of them related to caching.
Many sites might have plugins that attempt to cache pages and/or combined css and js files. If so, installing a new plugin might not invalidate the cache so that the Forum JS and CSS can be added to the page output.
The FORUM page should be excluded from any caching and the Simple:Press css and js files should be excluded from being combined with other css and js files.
After making these adjustments please make sure you clear your browser cache for the forum page (CTRL-F5 while on the forum page) and, if you're using a proxy service such as CloudFlare that you also clear the caches there.
I hope this helps.