Your problem is MT_SM Delivery from SMSC to MS after succesful SM_Submitting, as I understood. Isn’t it?
More details, pls, as follows:
1) What is the value of delays?
2)describe the situation under which this problem is occured?
3)What is the Reciepient subscriber MWD (mobile waiting data) status in HLR and VLR during appearing of this problem (status of MNRF,MCEF,MNRG in HLR and SM waiting status in VLR);