you can try this easy trick,
first trick, go to your portfolio list page. and save those portfolio.
most of time the problem is because permalink structure that not yet get refreshed.
or second trick, visit permalink structure on settings > permalinks. and simply click save to rebuild permalink.
if both of trick not working, please let us know. we may need to check right into your admin panel to see if something wrong with it.
also one more trick, try to change your permalink setting to day & name. most of time combination with trick above will fix the problem.