I did not notice that he customer uses UV-Inks. In this case the main part of the hardening is done in less than seconds, the missing % of the hardening needs normally only a few hours more.
If the hardening of the ink is not a topic, we should check if the ink is 100% suitable for the blister process. On the picture it seems that he connection is only on higher ink amounts not completely given.
Please double-check this with the supplier of the inks or test it together with the customer. The test would be easy, you need just a printed sheet with test stripes of 0%, 100%, 200%, 300% and 400% of ink coverage + blister coating on top. If the blister process works well on every ink coverage, only the blister process itself is left which we can check.