Firstly, and this might come across a bit harsh, but, you just might not be putting in enough effort. Its extremely easy to put weight on, but hard to burn it off, and it requires a lot of effort. That goes for both inside the gym and in the kitchen. If you aren't being as strict as you should (meaning, still eating junk food or doing half hearted workouts) then the results won't come. Its the same for a guy who wants to pack on a lot of muscle - if he doesn't train hard enough and eat enough of the right foods, he isn't going to add muscle.
You also need to be consistent. Its common for people to be known as a 'yo-yo dieter'. Someone that loses a bit of weight, then puts it back on and keeps going round and round in circles. You need to have a structure, so you don't jump up and down in weight. This could be anything from setting out your workouts, planning your meals or teaming up with friends for walks/runs/workouts.
Lastly, you need to be committed. There is no point giving 50% effort, because you won't get the results you're after and it would be a waste of time and money. Surround yourself with motivation (pictures of athletes, celebs, the old you), follow people who inspire you on twitter etc etc. It is hard work to get fit, but nobody is going to hand you a great body on a plate. No 'pill' or con diet will give you a great body for life, only you can do that. Put in the hard work and you will love your body forever.
If you need help with the nutrition side of things, pick up my get slim plan today.