Whatever you are using in that project, is overloading the system.... so it cannot process everything in real time.
It's likely to be you have used too many plugins that are placing greater loads on your system...
Some Tips....
1) Check your FX plugins and if possible, use busses for FX plugins that are duplicated with the same settings, on different tracks... so instead of say 5 FX plugins all working separately, you only use one, bussed to reduce the overall load.
2) Turn off Analyze mode in Logic's track EQ if used....
3) Freeze or Bounce in Place various tracks to reduce the load on your system... (This normally dramatically reduces load)
4) Reduce the number of FX plugins used or replace them with less CPU hungry versions....
5) If you are using Software Instrument plugins that have special CPU limiting functionality (Like Diva for example) switch the Plugin's performance mode to a lower setting like Draft mode in Diva...... (Check the manual for any 3rd party plugins you are using, as to if this capability is available for that plugin)
6) In a Software Instrument Plugin that is using a lot of CPU... try reducing it's polyphony to reduce the load...