I'd tend to guess that this might explain your problem:
Chaining output data
VTAM supports the chaining of outbound as well as inbound terminal data. If the length of an output message exceeds the outbound RU size, and the terminal supports outbound chaining, CICS breaks the message into RU-size segments and transmits them separately.
Your application can take advantage of the fact that chaining is permitted by passing a single output message to CICS bit by bit across several SEND commands. To do this, you specify the CNOTCOMPL (?chain not complete?) option on each SEND except the one that completes the message. (Your message segments do not have to be any particular length; CICS assembles and transmits as many RUs as are required.) The PROFILE definition under which your transaction is running must specify CHAINCONTROL=YES in order for you to do this.