GeneratePendingTransforms stuck every time
I think to solve your issue we're going to need a bit more of information. A couple of things you can try:


set a higher max execution time. I think normally the max execution time is 120 sec. and this sometimes blocks other transforms
try using the craft task manager plugin to remove any tasks that get stuck. 
try reindexing the images, remove all the cached items, check your folder permissions


That being said, I stopped using the image transforms Craft provides out of the box and started using Imager. Imager provides a handy way to do transforms in your template. Works like a charm and lots of handy options.