You are going to need a little more information and maybe tools. Also it might be help to explain how they are actually working.
I have seen 3 ways to wire a 3-way switch (power/line to the light with the two switches downstream of it, to the first switch with the light at the end, and power to one switch with the light in the middle). A diagram on these configurations can be found here
https://www.electrical101.com/3way-switch-wiring-using-nm-cable.html. If by install you mean ran new wire and added switches, you should check that the wires are correct; when we moved into this house there was one light on the basement 3-way switch that would be in the opposite condition of the rest of the lights when depending on one switch. It turns out it was wired to a traveler.
The most common mistake I have seen (and made) is at one switch swapping the line and the traveler. If when you say install you meant replaced existing (and working) 3 way switches, this is the culprit I would suspect. The best thing to do would ID every wire from the diagrams about and then make sure they are connected properly, this is where you may need tools to determine which is wires are hot under what conditions. (If you know which of the 3 ways it was wired and can ID one one wire you can infer the others). I'll admit that in the past with less experience, less tools, and more frustration, I have just swapped wires around in the boxes and tested the function; it takes longer, but it won't hurt anything and there are limited combinations.
3-way switches have a couple ways to indicated which wire is which; some have two screws that are the same color for the travelers, others will say "common" on the back for one wire (that is for the line in and light out), others have 3 screw colors (I've never understood why) usually the brass one is the common.
