We've had a few instances of issues with the heat-sealing over the past year.
We do take them seriously, and have taken steps to solve the problems each time as they arised, as promised at the time.
A timeline of events:
Several months ago we had a particular production run where heat-sealing was not done satisfactorily.
The cause ended up being that the heat-sealing machine was not given enough time to heat up properly before starting the packaging. As a result of that incident, we've retrained our production staff on the machines.
Shortly thereafter, as we switched packaging to the new resealable Aplix (velco-like) zipper, issues started to arise again. This time after research we found out that the heatsealing machine had to be set to a higher temperature than previously, to maintain the same level of strength in the sealing bond as with the previous bag designs. This was implemented immediately.
Lastly, another effect of the new bag design ended up being that the amount of space available at the top of the bags for heat-sealing to take place has been drastically reduced from the previous bag design. This was (mostly) the effect of the new velcro-like zipper being wider (taller) that the previous resealing strip. Thus the margin of error was reduced, which means that more care and effort had to be taken by the production personnel to seal straight, and in the correct place. This is also the cause of a similar reported issue, where the heat-seal is performed too low, and seals below the tear-line.
We ended up changing our production protocols so we are logging which employees are sealing which batches at which time, to introduce more accountability on that front. This allowed us find, retrain, or in some cases even fire some employees that were insufficiently diligent during heat-sealing.
As you can see, it's not a single issue reoccurring over time, but separate issues with the same end result: A faulty heat-seal.
I understand that as an end consumer it can be frustrating when this happens, but I do hope you believe we are doing our best to avoid such issues. Please email support when this happens, as we're more than happy to resolve issues like this to your satisfaction.
With regards to the frequency of this occurring:
We send out thousands of bags every day, and we get a complaint about a popped bag I'd wager around thrice a month. So it's not a wide-spread systemic issue.
With regards to the fulfilment personnel noticing the issue during pick-and-pack:
In our experience with issues like this, the heat-seal is in a weakened state. When we ship it, the bag is fully closed, but as it undergoes the rigors of international transport, the bag 'pops' as it is put under stress and the heat-seal can't maintain.
This hypothesis is substantiated by the fact that there is a clear correlation of customer reports of heat-sealing issues, and the amount of time the parcel spent in transport. (i.e. the further away from The Netherlands, the more likely a weak heat-seal pops).
I hope this answers some of your questions, and assuades some of your fears on the prevalence of this issue. Please let me know if you have any questions or suggestions with regards to this!