When you say that the compressors can also run at 208 volts, do you mean that 208 volts is the absolute lower limit, that is it wont run at 207 volts, or do you mean that they will run on a nominal 208 volt system, which could be as low as 190 volts.
If the compressors are 230 volt nominal, with an absolute lower limit of 208, then it would appear that the wrong compressors have been specified.
230 volt compressors on a 208 volt nominal system=unlikely to work reliably.
The fact that they work at 208 volts, but not at 206 volts, does perhaps suggest that they are 230 volt nominal, with an abolute lower limit of 208.
The options would appear to be new compressors suited for the available service, or a new service to suit the available compressors, or voltage increasing transformers.
If however the compressors are suited for a nominal 208 volt service, and you have a 208 volt service, then they should work fine.
The fact that they dont suggests either a low utility voltage when heavily loaded, or excessive voltage drop in the installation.