A car needs plenty of Ooomph! to get the car started using a battery. A car bettery is designed to give it that burst of power, and not for long lasting slow drain power. Also, there are other components on the car that also use the same batter to run when starting, so there's a nice bit of draw from it when cranked.

So, it's when you say that you use a jump pack or extra battery suggests that the battery you are using is not up to the job. Is it a new one? 21 ah sounds like too small a battery for the job, but it has worked before, perhaps theres something else on the car draining the power needed to start it.

Now, ther only other thing is that if there is a component on the car ( like the fuel pump) that needs the full 12v from the battery, but is faulty, then maybe that's worth considering.