The huge number of memory types available today makes the selection process critical. You must purchase memory that is compatible with the memory controller and memory socket(s) on your motherboard. Additionally, although people will say that this is not critical, I recommend ensuring that the memory sticks that you add to an existing configuration are in all respects the same specifications as the ones that are already installed.
I looked up your computer. It takes PC2-5300 memory sticks. Make sure that the memory that you added is this type. More importantly, make sure that all of the memory sticks in your machine are of this type.
http://www.pcmemorystore.com/HP_Comp...-6715B-ram.htm
I didn't look up an on line user manual if one is available so the next thing is speculative. Many motherboards have BIOS settings that must match the hardware configuration. These days the BIOS is usually self configuring but there is some chance that the motherboard did not reconfigure its BIOS to adjust for the additional memory.
Make sure that your motherboard is set to safe specifications. In other words, don't overclock until you have everything working at normal speed.
Some of the newer memory sticks require that you set the voltage higher than motherboard defaults. Read the specs for your make and model of memory. See if the manufacturer tells you to increase the voltage, even for rated 'normal' operation.
And lastly, I wouldn't have bothered to add memory just because my manager would approve the purchase. It is always a good idea to keep what works until you have a demonstrated need to make a change.