I think it is because of a Job setting on the FTP tab. I have here screenshots of both AWI and Java Client.In AWI,. under FTP tab, you have job settings and there are three settings that can be changed.
The "number of transfer threads" Is by default 2. Once you change that , you should be able to transfer more files.
https://us.v-cdn.net/5019921/uploads/editor/xh/xywcznll6v2u.jpg" width="802">https://us.v-cdn.net/5019921/uploads/editor/6o/1gztrzgulv4z.jpg" width="937">
Above is java Client