Sometimes it is not easy to detect the P-wave because these waves carry little energy, most of the ultrasonic energy is propagated as R-waves.
You can avoid this problem by using a through transmition configuration.
usually, high w/c ratio when concrete is overfinished can lead to this results.
It may be possible that you are not catching the P-wave and the unit detects the R-wave, which is 60% slower than the P-wave.
