New Ceilings query

Hi
Not sure if you can help.

We live in Perth in a 28yr old 3b/r house and have just had new ceilings installed. The company replaced like for like as the old ones were bowing alot.

We have uncovered lights in the house, and have spotted a number of screws popping in the ceiling, and what, to us, appears to be lines where the sheets meet. We have contacted the company and they have come out a number of times to plaster the screws, sand and then paint. This has been going on for 4 weeks. I wrote them a letter asking for someone to come and inspect the installation and determine what is causing the problems we keep seeing. They replied that it is only aesthetic finishes and are not structural and that the problems with the old ceilings was the fixing methods used . They have installed the ceilings as per Australian standards, which were all quoted, and referred to glancing light as a reason for not having to rectify problems.

My query is - should we worry about these popping screws, or leave it. We havent paid a cent so far, but their reply letter has now asked for the payment.