First and foremost, follow our installation instructions. The most common causes of problems related to wallpaper seams are improper wall surface preparation, improper adhesive application, and / or extreme conditions in the room environment.
Lets assume that you have done everything correctly with your install, but the seams are still opening up on the wall. Now you may wish to read the attached document. This document describes a procedure, sometimes referred to as "Velcro" technique. This does not involve using actual velcro strips, but it is in fact a pasting technique used by skilled installers on stubborn seams. This technique can be helpful in the rare condition where the seams want to curl and the paste is not strong enough to hold them down. This potential exists on all products but is most commonly experienced on our heavier weight materials. Could be a wide array of conditions that you are fighting here, including atmosphere, wall condition, product weight, adhesive batch strength, etc. 
In addition to the "Velcro Technique" - in the attached document we recommend switching to a stronger adhesive and a different primer. Here are the instructions ( which is taken from the attached document).
Many times seams on walls open up because the drywall underneath was damaged during install. During install the wallcovering is slit and if the blade cuts through the wallcovering into the drywall, this could result in the paper surface of the drywall releasing at the seam. The best way to prevent this is to use a seam cutting blade which protects the drywall underneath. Here are two similar products we recommend that will help. They allow you to set a metal plate behind the layers of wallcovering that are being cut, essentially shielding the wall from contact with the knife. YELLOTOOLS BODYGUARD KNIFE or ADVANCE SEAM BUSTER
In regards to those applications and jobsites where seam performance is brought into question on products where seam performance can be sensitive, the following steps can be utilized to aid the install of such products:
1. Roman R35 primer is satisfactory for average wallcovering installs. However, where seam performance is brought into question, R35 is not an optimal primer in these applications. The preferred products would be Zinsser Shieldz for new sheetrock applications and Zinsser Gardz for renovated sheetrock applications.
2. Heavyduty clay should be used for these types of applications. Roman 732 HD Clay is one such product. Roman 774 Clay strippable is also satisfactory, but 732 would be preferred.
3. No paste dilution should be allowed, and no usage of a pasting machine.
4. The product should be pasted by roller and “booked” a minimum of 10 minutes. Longer booking times are allowed and can be beneficial to allow the adhesive to soak in and stabilize. Increase booking time as needed.
5. Employ the “Velcro” process for paste installing wallcovering. Michael Krawiec of JATC, who has demonstrated this at the 2015 WA Tech Track program in Chicago – as well as several other experienced installers from the NGPP have advocated this process. This process utilizes the HD Clay booking of the product, but adds the step of prepasting the wall with HD Clay in the area where the seams will exist. The HD clay should be rolled on the wall and allowed to dry. The dried clay on the wall provides an additional bond that reinforces good seam performance.
6. Do not work the seams heavily after the wallcovering is applied to the wall. Apply gentle and even pressure for smoothing. Heavily working the seams can push the wet adhesive away from the seam area degrading the seam bond. It can also stretch the vinyl and encourage snapback – which mimics the appearance that the material is “shrinking” and pulling back at the seam.
7. Use a seam buster knife for providing the overlap, double cut of wallcovering strips.