As we know Saurik is the Fonder and creator of Cydia, has updated the cydia substrate to 0. 9. 5013 to support it with iOS 8. A Chinese team called themselves Pangu, recently released the iOS 8 Jailbreak but it not comes with the Cydia as software did not support iOS 8.Now that Cydia Substrate is updated, most tweaks are now run on iOS 8
Cydia Substrate is that which makes Cydia tweaks able to run on your iOS, act as support firmware. According to Saurik iOS 8 pangu has a bug which does not load substrate after rebooting iOS Device. There is a need to respring the device after reboot to load Substrate and install tweaks.
Note:- it will not work before a reboot and a respring, after this it will goes fine. You will have to respring by SSHing to your iOS device and type this killall backboardd.
If you have updated your iOS 7 to 8 and already have this pangu jailbreak then Refresh the changes in Cydia and check out this update