We study the retrieval efficiency of stored light pulses based on electromagnetically induced transparency in multiple simultaneously driven Lambda-systems. The light pulses are stored in coherences between different Zeeman states of laser-cooled atoms. When the stray magnetic field from the environment is minimized by compensation coils we observed a smaller retrieved probe pulse amplitude than for a small externally applied magnetic field, i.e., a seemingly shorter coherence time. We identify this effect as a beating of several coherences due to a very small uncompensated dc magnetic stray field. By intentionally applying a small magnetic field larger than this stray field we were able to increase the retrieved probe pulse amplitude up to five-fold to the value determined by the true coherence time of our system.