We can't know. You have not provided any proper system information or other technical details like project settings and your terminology is fuzzy. eitehr way, ask in the Premiere forum.
We'll need a bit more info as mentioned above ...
-OS/CPU/RAM/GPU/vRAM and driver details.
-Specific program versions ... "2014" or "CC" or "Latest" don't count. Give us numbers off the "About" page.
-Footage codecs & info, what cameras they come off of ...
-sequence settings ... and are you blending footage & frame-rate & sizes besides using 3rd party plugins?
-which specific plugins are you using, and if you try the same thing through PrPro's masking tools do you have problems (trying to see if it's just the plugins or if your Prpro's masking stuff is wonky)?