Hi,
Most likely you have your sample rate set to 2 seconds and the memory on the device is full. The Pro Plus in particular has a very limited memory, especially with a small sample rate, and ~5 dives is not uncommon.
Note that the Pro Plus also has a "quirk" where it will show you more dives on the device itself - it records the general details of a dive (date, max depth etc) separately to the actual profiles.
This is covered in more detail in the help:
http://mac-dive.com/help/index.php?titl ... y_dives%3F
In particular the last section, "but I can see them on the device", this all pertains specifically to devices like the Pro Plus 2.1.
Cheers