I sopose that your 1800MHz sektor curring only HO traffic.(HO from 900 to 1800)Could you see in stats that you have some some sdcch trafiks on 1800?
a)check if you have SDCCH configured on 1800MHz.
b)check LB , but if during daytime there is a traffic , than it can not be a problem. anyway chack
c)check the BTS parameter – “not allowed access class”. May be a mistake.
d)If you are playing with C2 criteria on 1800MHz, then there is possible situation when C1 neigbour C2 (in your case 900 sector), then MS need some time for reselection.During this time the mobile not reacheble because of C1