I have recently bought a new Mac which contains Yosemite. Unfortunately i have problems with some of the programs I'm trying to run so i want to downgrade back to mavericks. i have constructed a USB flash drive that consists of Mavericks, however every time i select to boot from the Mavericks install USB a circle with a line through it appears on the screen. is there ways past this?
It really depends on exactly when your Mac was made. Right down to the week. If the hardware contains things that Mavericks doesn't know about, then it won't run Mavericks, period.
Your model may have been available before Yosemite, but that doesn't mean it's got the same components - just the same specifications. Foxconn might, for example, have run out of Mavericks-compatible USB bus chips but Apple said go ahead with the new chips and a driver will be included in Yosemite.
If you are sure you created your bootable USB correctly (it boots other computers into Mavericks) then you are stuck.