What if I keep failing at the same weight?
You take a step back in order to take two steps forward. Here’s how: if you fail to get 5×5 on an exercise three workouts in a row, lower the weight by 10% for that exercise only. This is called a deload. And it will give your body extra rest as well as avoid mental plateaus.

So if you failed to get 5×5 on Squats with 100kg/220lb three workouts in a row, deload to 90kg/200lb next workout. This weight will feel light but take it seriously. Focus on proper form, lift fast on the way up and breathe correctly. Add 2.5kg/5lb each workout until you’re back to 100kg/220lb. You’ll make it.

Note that you only deload on the exercise you failed to get 5×5 three times in a row. If you plateau on your Squat, but not on your Bench Press or Barbell Row, then you only deload on Squats. The other exercise keep increasing by 2.5kg/5lb each workout.

Best is to download the free StrongLifts 5×5 app for iPhone/iPod or Android phones. They’ll tell show you how much weight to lift on each exercise if you keep failing at the same weight.

Diet should be fine.

But I usually do my SL in the morning before work (7:30am).

That shouldn't pose any issue because I too did SL 5x5 with Intermittent Fasting and workoutat 8.00 am.