With an 8 month-old, I'd start by offering exclusively the "P" fruits- prunes, plums, peaches, pears, aPricots, and cherries (like the other "P" fruits, cherries belong to the genus Prunus, and in my experience they are effective at getting things moving!).
I'd also see your pediatrician. Depending on the severity of your LO's situation, you might want to discuss medications. Until you can see the ped., I suggest keeping a "poop journal." It can be hard to accurately recall how long it's been since your LO last went. 3 days or 5? 5 or 7? 7 or 9? And a journal can help you see patterns you might not have noticed, otherwise.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"