Masonry constuction is supposed to last a lifetime or more don't let anyone tell you different!
You thought your flagstone patio or walk was supposed to be durable, right? You can't believe that you've only had it for a handful of years and all the joints are cracking and the tiles are coming loose. Your contractor or builder won't take responsibilty for it, right? I imagine you're being told things such as, "you're going to have to contact the masonry contractor" from your builder, or something like, "you just have to get it repointed every so often" from the masonry contractor....and round-and-round you go. Are you tired of it yet?
A good portion of my work is removing another mason's shoddy flagstone work and redoing it properly, and when I say a good portion I mean most of my work. I have seen the same story over and over again and it never gets any better, the wrong cements were used and the applications were poor at best. It's easy to make that patio look good long enough to get that check and disappear and this is unfortunately the most popular school of thought going around out there, I'm calling it an epidemic...someone's got to! Once upon a time guys used the right mix and set every single piece at least twice and that work is still up. You can go through any neighborhood in the city and see for yourself, those patios and walks are 80 plus years old and still in good shape. You deserve that same product, especially at the price this stuff goes for.
Tip #1. If your mason isn't setting every piece at least two times then your patio is going to come apart sooner rather than later. Understanding the process, the concrete pad is poured first and must be allowed time to cure before the setting of the stones. When it is the process is to lay rough height mortar bed (proper mixture with the right moisture), set the stone and tamp it down to the precise level needed. This same stone must be picked up again and that mortar bed inspected for soft spots (these can be seen at a glance with a properly trained eye), any soft spot need additional mortar and the piece can be glued for the final set. If that bed isn't solid then it will take in more moisture than it should, when it's cold enough to freeze that water it will expand and flex the stone...then the dominoe affect occurs and your patio is a goner.
Tip #2. If your mason breaks out the pastry bag and squirts your joints in, your patio will come apart quickly. The "cake bag" as we call it is exclusively for "vertical application". Top reason, because flagstone is a "flat application" or on the ground, it is vulnerable to the moisture. Where water would mostly just run off the stone front on your house, it's sitting on your patio and a slight pitch is drawing it down eventually and so my point is that you have to use a mix that is diluted with water to get it through the cake bag...diluted is always weaker and using a weaker mix on the most vulnerable type of masonry doesn't ever turn out good. Second top reason, a cake bag is a particular thing believe it or not. You have to use a specific mortar mix to have it fluently slide out of the bag and that mix is the absolute worst thing to use on a flagstone joint.
There is an aweful lot of truth in the saying "you get what you pay for" and if you're hunting around for the cheapest price in this field then outcome will more than likely be not so favorable for you. Is it cheaper to get the cheapest guy and then pay someone else to redo it...or is it cheaper to pay the right guy to do it the right way at a decent rate? Be careful though because that doesn't mean the guy that cost more is going to do it right, I would encourage you to get a reference list from your potential mason and to follow through with the investigation, make the calls and make sure it's not his Mom you're talking to!
Flagstone is the most meticulous operation in the masonry field and if the mason doesn't care enough to do the aforementioned manner of application then the homeowner is the only one left to pay. Ask him how will he stand behind his work!