Check out that calorie burn!!
Last week I ate 8793 cals and According to my Fitbit I burned 20347 calories, creating a deficit of 11554 calories. That deficit equates to about 3.3lbs of human body mass. add on to that water weight and general inaccuracy of my food input (if I don't know the weight of something, I over estimate), and you can see how 6lbs racks up!
I essentially lost 1lb for every day I was in the gym 
Bloomin' heck. That is amazing.
According to endomondo, my epic 55km birthday bike ride along the Kent coastline burned 2068cals, which I was mighty impressed with and saw as a one off mad thing, especially in contrast to my usual 365cal 4.6km runs. All this turned upside down by finding out that some people can burn 2000+cals just in their daily trips to the gym!
Please describe what kind of things you do in the gym and how long you are there for. I am a regular user but burn only around 500 calories in my workout and then something similar in an exercise class, over a period of 1.5 to 2 hours in total. Really need some motivation and ideas to burn more.
I'm there for 30 mins. Monday: kettle bell class 45 mins, around 600 cal burn.
Tuesday: rest day (3 mile walk with dog) Wednesday: swimming for 30 mins
Thursday: cardio: couch to 5k program for 30 mins Friday: swimming Saturday: cardio
Sunday: swimming.
The calorie burn is from my Fitbit, it's for the whole day not just my gym time.
Doing 2 hours in the gym isn't going to help you lose weight. I'm guessing you're just smashing the cardio? Have a look at HIIT training for cardio, burns much more efficiently than just running Etc.
You also need to
Do strength training to build your lean muscle. Lean muscle burns calories 6 times faster than fat, so the more you have, the more efficient your calorie burning engine is. The mistake most women make is to just smash out cardio at the gym because they think it will help them lose weight when actually you're much better off
Combining cardio and strength
Also I forgot to say, if you're managing a 1.5 hour work out, your intensity is far far too low. 30 mins should absolutely knacker you. This is most likely why your burn is so low. You have 4 heart rate zones: resting, far burn, cardio and peak. For your workout you should be at fat burn and above for the whole time, if you're managing 1.5 hours you're most likely in rest - fat burn for the most part.
Gym time isn't about being there a long time, it's about pushing yourself I reccomend watching the biggest loser on YouTube, check out how hard those guys push themselves to get an idea where your intensity should be
Very interesting comments about what best to do at the gym. I enjoy swimming and just swim until I am really tired - which takes about 1 hour more or less constantly swimming breast stroke. I wondered if you knew approx. how many calories that might be ?
Thanks for that although I had stupidly misinterpreted your comment as reflecting calories burnt at the gym (I don't have a Fitbit ) My own routine doesn't sound too dissimilar (my gym workouts are 30 minutes cardio plus stretching in Kinesis room) and then a variety of exercise classes ranging from HIIT, yoga and Pilates to mini trampoline and fitball. I go 4 times a week. I think, based on your experience I shall try more swimming and have recently added 2 long weekly walks into my schedule which seem to be making a difference. In any event variety seems to be the secret and at least it staves off boredom too!
That was my fault I think - I only know my calories for runs and cycles due to using a basic gps-based app not a wizardy fitbit. Good to know all the other activity during the day adds up like that
Why aren't I losing?
Lost 1 stone! :) 
Why am I so hungry?
Why aren't I losing weight.
Why so difficult?
