Biscuit hardness problem

Hello everyone,

I have a problem with hard biscuit production, similar to petit beurre.

We are working on a diesel oven with 3 burners, each has separate temperature control.

upon cooling the biscuit, it becomes extremely hard to bite through, and when it does it tends to break from different spots, not where the teeth are biting.. Let's say like breaking glass, metaphorically speaking !

The color of the biscuit is perfectly fine, it's not burned or anything, just becomes hard when it cools..
is it the sugar? or the suryp?

We did make changes to the recipe, and found some improvements, changes were as follows:

We are using regular white sugar, each batch contains 100 kg flour, we started with 26kg sugar per batch, and adjusted the quantity to 15 kg per batch, it did get better noticeably, but not like the products that we find in the market.

We also use invert sugar, hand-made and not bought, what are the optimal quantities? we are currently adding 5.5 kg per batch.

As for glucose syrup, we tried using it a couple of times.. Didn't see much improvements though and so we stopped..

We also changed more than 8 sources of wheat flour, all gave the same result.. 

Anyone ever encountered such a problem and knows the reasons behind it?