I had done quite well on 5:2/4:3 from Nov last year until about April, then I went a bit off-piste until August and gained about 9 lbs.
Then I said enough was enough and started back in earnest and have lost 1 stone 5 lbs since August.
For whatever reason, my body doesn't seem to lose weight easily - I exercise (hard!) 3/4 times a week and don't always lose every week so I have found it is crucial for me to check and log my calories every day on My Fitness Pal - I stick to 500-600 on a fast day and aim for 1200 on a non- fast day (even though my TDEE per the tracker is double that!). It's really useful to have the history of what you have eaten on MFP so you can see what's worked well and what hasn't.
Hope this is helpful - feel free to check out my tracker!