In essence, this is happening because the source device feeding your dCS product is outputting a signal which has very inaccurate clocking. dCS products use Voltage Controlled Crystal Oscillators (VCXOs) with a sync range of +/- 300 parts per million. If a source is outputting a signal which is outside of this range into the dCS product, the dCS unit will continuously drop and re-lock to the signal. This is because the VCXO cannot physically lock to a signal this far out of range.
It may be observed that some devices can synchronise to the same source without issue, however this should not be taken as a direct comparison. Many other devices use different methods of synchronising to a source. For example, SPDIF receivers and Phase Locked Loops typically use a Voltage Controlled Oscillator (VCO) instead of a VCXO, which has a much wider range that it can accept but equally it introduces more jitter into the signal. This compromises the performance of the system compared to a VCXO, whether the source is properly clocked or not.
We do not feel that sacrificing the performance of the system to bring compatibility with sources outputting out-of-range signals is a compromise that should be made in a dCS digital system. As such, we opt to retain high-quality clocking with a slightly more specific sync range. While this means you may get dropouts using sources outputting inappropriate signals, it retains the expected quality from the system when using a well clocked source.