Help with ModBus RTU Master-Slave: SimpleModbus [SOLVED]

Good evening,
I am developing a monitoring system through an Arduino Mega master connected to a slave PLC.
Communication OK.
But I noticed in my application that when I used the PACKETS in sequence, the system collapsed, read out, lost connection, I discovered by chance that when I skip the PACKETS the values are read and there are no connection losses.

I ask you whether this is normal or what could be causing this?
The way the system works is to have a delay to carry out the entire reading of at least 20 seconds.
Can I, for example, decrease this time?