Can't say I've noticed. Try marinating the meat first? Let it sit in some lemon juice or white wine vinegar for a few hours beforehand, this will tenderise it as well as giving it an authentic tang. With chicken, try thigh and/or leg rather than breast - much more moist and more flavour - better still, leave them on the bone, this will make them even more tender and flavourful.
Of course, if you did all that and it was still dry, I don't know what else to suggest!
I generally avoid using chicken breast - second Backdrifter's suggestion - and use a jar of paste instead, adding water and sieved tomatoes (bottle of Tesco's Passata is very convenient) to simmer. Results are pretty good with all types of meat - if other than chicken, a little fat on the meat makes it nicer.
When i use these sauces i cook everything slower and for longer than it states on the jar.
When all ingriedients are in the pan i let it simmer for a while - at least 45 minutes, i think it helps all the flavours infuse.
i always make it this way and my chicken is never dry, I always use breast meat.