Most likely the 888s are set up to require a "tone" to break squelch. This is very common for a radio that's been used with repeaters, and almost all repeaters use CTCSS, aka PL tone, to minimize interference.
The "mon" button disables squelch momentarily, allowing the signal from the UV-5R to get through. You can avoid this by finding out what tone the ...
I am guessing that you programmed a receive tone requirement. Try programming only the transmit tone and leaving the receive tone not required.
I don't have Chirp available right now so I can't give you the names of these fields.