tcp retransmission & tcp out of order and tcp frame missing

Dear Gurus, Sorry if i sound stupid but i am very new to networking.My application work as ESME for the smsc and it seems that recently we observing huge delay in ack of submitted message or ack not coming at all.I can see in netstat output of linux send queue is increasing hugely.Taken a tcpdump in which i can see to much [TCP Dup ACK],[TCP Out-Of-Order],[TCP Retransmission]

Also i see in wireshark Smsc send mesg with

seq:1 ACK:1 Len: 51 -->"213.132.40.140 -> 192.168.40.8 SMPP SMPP Deliver_sm"

but we reply by

192.168.40.8 -> 213.132.40.140 TCP 42758 > dynamid [ACK] Seq=4141 Ack=52 Win=128 Len=0

That means are we losing frames.Pls help