I thought I'd post this as it was one of those problems where after numerous discussions with support, there didn't seem to be any fix.
In fact, every support member I spoke to, although helpful,had a different take on it. At first they said it was a logged, known issue, then it was an issue only a couple of people had reported then it was an issue that they hadn't seen before.
Anyway, the issue was when creating a smart task then enabling it I then tested it out by importing a record which met all the conditions of the smart task but nothing was happening, it just wasn't kicking in. Simply disabling the smart task then enabling it again strangely fixes the problem. I created well over 20 smart tasks today and each time did the above and received every email I should have received every time (whereas before realising this, I must have tried importing at least 15 dummy records to no avail!)
The same goes for editing a smart task. If you need to do so, ensure you disable it first, do your edits, then re-enable.