MSL5052S2

[ZoiloG]

I have a three Msl5052S2 with pass-throught(master-slaves) and several times in the master library I see the message:

"Some modules have not been detected during the physical inventory.
Some possibles causes are:
a)Defective opto sensor in the module.
b)The modules is not properly atteched to Xpresschannel hardware."

Several times after restart the slaves are online.
Why can be?

Thanks