I've only been having this problem with Die Rise, it doesn't seem to have effected the other maps.
Check your bandwidth - go to Options then press select. It needs to be about 900+ kbps to play solo. For some reason the bandwidth on Multiplayer is considerably higher than Zombies, which seems to be the main issue here. If it's low on Zombies, try this - go to Multiplayer and sign out, then go to Zombies and sign back in. Your bandwidth should have increased, then try solo it should work