In case the "Programming Mode" is set to "Power Cycle", the programmer will be unable to program the device. Since, it will be unable to acquire or detect the device by turning power off and on. Power cannot be turned off since an external source (12V DC adaptor) is powering the device. If you want to program the device using an external power supply, set the "Programming Mode" to "Reset". In this case, the programmer will detect the device by manipulating the Reset line.
In case, the MiniProg is unable to power the board, kindly do make provisions for supplying external power, and as mentioned above setthe "Programming Mode" to "Reset".
I hope this answers your problem. Do let me know if issue still persists.
I will try this and let you know