Images of only small breakpoints created

[NeilJS]

Similar to issue 117: #117

I have this exact problem.

In testing, I use the example to force width to 400 - this generates the 480 cache ok. Forcing to >480 is not triggering the other default breakpoints (eg 768, 992 etc).

I created another breakpoint to find the point of failure, which was 639. So forcing the screen width <= 639 worked but anything over that did not trigger any of the other breakpoint sizes.

I'm on wamp, testing with both desktop and mobile; upped my php.ini memory to 512M, no joy. Very odd! Maybe it will be ok on the live server but if anybody has any ideas, would appreciate it, thanks.