Things like this will happen if the VST is 32 bit, and you are using a 64 bit version of S1. Also, in the locations tab in preferences, make sure that you check off Rest Blacklist, and also scan at startup is selected.
To verify if the 32/64 bit is getting in the way, install the opposite version as you have installed from your presonus account, and see if the plugin shows up then.
Last possible scenario, you are using Artist version that does not support 3rd party vst without purchasing the add on.