Fitbit - Have I been doing it wrong?!

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Hiii :)

So its been 2months since my last recorded loss on the scale :'( Despite exercising now more than I have done ever in my life. **Frustrated**

My stats are: Female, 23yrs, 5ft8, 136lbs!

I have a fitbit which I wear 24/7 unless I forget to put it on but that's not very often. I also use a HRM for my exercises and log that to MFP.

I have it set up to be in a 500kcal deficit a day on a medium intensity plan on fitbit. I have it set up for negative adjustments on MFP so it steals my calories the less active I am! I haven't been eating back my exercise calories and I'm thinking this is where I could be going wrong? If fitbit already accounts for a 500 deficit then should I be eating back my exercise kcals? I roughly burn 2100 kcals a day according to fitbit and more on a highly active day. So I should be aiming to eat around 1600kcals PLUS exercise calories back?

TIA :)