Glonass is tricky for RTK because of the inter-channel biases. All satellites transmit on slightly different frequencies (compared to GPS or Beidou) and that results in receiver-specific delays in the front-end. Ability to get fixes with glonass might depend on the receivers in use. It is best to use Glonass ambiguity resolution when using a pair of Reaches. You can also use Glonass satellites, but not include them in AR by opening “advanced settings” and changing “GLONASS integer ambiguity resolution mode” to “off”.
You can have shift in fixed solution because of multipath, are you using a large metal ground plane under the antenna?