There are many possible causes. Without sight of it, it's not possible to tell which may apply to your project. A couple of the common ones: 1. component placement is too close (or copper clearances are too wide to allow routing to complete;  2. design is too complex for the EasyEDA Autorouter to handle.

@andyfierman Ok thanks anyway, my design is not yet complete so I'll plod on for a while and try again.

Unfortunately the autorouter still needs more development work. Personally I don't trust them anyway because even with the very high end commercial autorouters there's still huge scope for them making a mess of signal and power integrity and EMC. They can create a mess of even a simple board in the hands of newcomers to PCB design and in the hands of experts, it probably takes more time to set them up and then check the results than it does to manually route the board. :)

@andyfierman I think you're right about needing further work, for me it's great to get the board routed for me then I can tweek it to suit my paticular needs. It's been successful for me up till now but because this is my first foray into SMD components and miniaturisation, ATMEGA328P-MU etc. starting from scratch is a bit of a chore.