AVRISP-U AVR USB Programmer gives Device Not Responding error. Why?
First, check that the 10-way ISP lead is inserted in the correct header on the target and that the target board is powered. Make sure you are using the correct lead - the 10-way lead supplied with JTAG AVR Emulators is different. Now try wiggling the lead to see if it has a faulty connection.
If you are using your own target circuit, make sure it fits with recommended ISP circuits - AVR ISP Circuit Guide
AVRISP-U software will try and Auto-detect device and set the correct programming speed, which depends on AVR clock fuse settings, target clock speed and voltage. So, unless you have a very slow clock, low voltage (1.8V) or unusual ISP circuitry, slowing down programmer will not help
If you want to try slowing down the programmer, Click Setup Button and disable Auto Speed Set and select Programmer ISP speed manually.