Digital BW, The Print

Have come across a problem that I don't understand and does not
make 
any sense.
I have a CIS with MIS FSN inks with the Piezography BW software. I 
did not use the printer for about 3 weeks, when I tried to print the 
black nozzle was not firing. I did couple of cleaning cycles but 
then all the lights on the printer flashed. My printer is still 
under guarantee so I phoned Epson. They got me to reset the print to 
no avail. Under the guarantee they sent some one to fix it. I took 
the CIS off put the original cartages in then every thing worked OK 
(had a problem with paper feed so still got Epson to have a look).

Epson exchanged my printer with a new one. Have to say Epson back up 
is second to none this is not the first time I have dealt with them.

New printer worked OK put the CIS back on again the black nozzle is 
not working. I have took the CIS off put original cartages in and 
the blacks OK.

Every time I have taken CIS off I have put plugs in and nipped the 
tubes closed so no air has got into the system. There is no air in 
tubes, cartridges are full if I take plug out of cart the spike is 
coved in black ink so as far as I am aware the CIS is in perfect 
working order.

At this moment in time CIS is off the printer and original carts in 
and all is working well. Could any body suggest my next move? Is it 
worth putting the CIS back on doing some purges and cleaning cycle 
and leaving for an 8 hour stretch. 
Should I put a cleaning cart through before putting the CIS back on? 

Has any one ever had this sort of problem as I have said it makes no 
sense as all seems OK?

Kirk