Just wondered if anyone on here could help me...

I'm currently looking for a PhD, and have been contacting academics in the field that I wish to study with some quick questions about the potential for PhD study in the future.

On all occasions I've been very polite, enthusiastic and have given a brief background on my previous study and explained that I really want to start a PhD quite soon.

However - I'm having one major problem. They're all, basically, ignoring my emails. This isn't an isolated case, this is across four different universities... has anyone else encountered this problem? And does anyone have any tips for encouraging academics to reply to correspondence... as I'm becoming a bit disheartened at the lack of responses that I'm getting.