how to find the time sync?

when i use the single spl_lio_sam_imageProjection node to run, the console show "Waiting for IMU data",i got that because " stamp2Sec(imuQueue.front().header.stamp) > timeScanCur " , and my imu data ,point data time not synchronous,  but when run the all node of simple-lio-sam.the problem disapper, i want ask why ?