It's not uncommon to feel like you get to a point in your training and then struggle to move on - progression is not necessarily going to be a straight line when plotted on a graph. Plus there are also many other factors that can influence your training such as illness, workload, family life, etc. The Great Run Training plans follow tried and tested methods and gradually increase the amount of time/distance that you run as your fitness improves. It could also be that you are setting off too fast. Always run very easily at the beginning of your run.