This happens commonly with older or budget TV sets. I guess what has happened is you have set the system up, tuned the TV sets in and some are working and some aren’t (or maybe just one isn’t)?
The one(s) which are not working have saved the modulated channel into the EPG and location is right, the channel name appears but the screen is blank and you might have a message along the lines of ‘format not supported’ or similar.
This means that the TV set is not compatible with the video compression/format which is generated by the modulator. The TV set needs to support H.264. The screen with the blank screen will be MPEG2 only and will not display video from an HDMI modulator.
Sometimes the audio will work but don’t let that get you excited, this tuner is not compatible!
The options now are; replace the TV with a newer one or install a set-top-box such as a Freeview box or an Antiference DVB01 mini terrestrial receiver and connect to the screen via HDMI or CVBS and not use the TV tuner.