First, verify that the label printer is turned on and that it is connected to the network. Many printers have a Wi-Fi light that will indicate if they are currently connected.
Second, verify that the iPhone or iPad is connected to the same wireless network as the label printer. Sometimes the iPhone or iPad will choose a prior network to connect to if that network is received first. This can be prevented by selecting “Forget Network” on the device for the other networks.
Finally, sometimes if the network is congested it can take up to 30 seconds for all the printers connected on the network to reply. Therefore, simply trying a second time to select the printer will usually resolve this issue.