In what way is it hard to start? You're not explicit. Needs to crank a lot, or only turns slowly on the starter motor, or what?
Drive the car, open the bonnet, lock down the bonnet position detect button to fool the car into thinking it's closed (I use a crocodile clip underneath). Then lock the car (with the bonnet still open) and walk away until it's gone idle and then a bit (about half an hour). Measure the voltage at the charging/jump-start posts in the engine bay. What's the voltage? And what's the ambient temperature? The figures should show if you have a battery issue (new batteries can be duds). Wait a few hours and re-measure the voltage. It's not accurate but it should give you a good idea if you have a leak. It shouldn't have dropped much between your two readings... You can get charts on the Internet to give you an idea of what the voltage at rest should be for your battery type and ambient temperature at various states of discharge.