Trying to access OS X Utilities screen on start up CMD+R

I am trying (without luck) to reformat my MacBook Pro. I do a lot of photography and it is becoming sluggish. I have went through the following process:

*deleted all non-essential files/images

*made a back up on TimeMachine to an external Hard Drive

*made a Yosemite reboot disk to a flash drive

Now, I'm trying to get into the OSX Utilities screen by holding down "Command" + "R" and it will NOT do anything but reboot as normal. This step is essential to being able to totally wipe my MacBook clean. And, I'm in a time crunch...I have some photos that i need to process!

If anyone can provide some advice, I would greatly appreciate it.

Thanks!

Jacki