We are infusing a small flat sheet as shown in the picture but we keep running into a problem with the infusion using a lot more resin than expected.

We are using 2 layers of 200gsm CF with 10mm easycell infusion grooved foam in between. We estimate that for 1 sqm of the laminate, we will need 400 grams of resin (for the 2 layers of 200gsm CF) plus another 700 grams for the infusion mesh, with an extra 100 gm for the lines.

The infusion starts nicely and at a decent speed, but the problem is that the resin runs out with 50% only of the part infused or even less. We eventually had to put 3 times more than amount we calculated until the resin finally reached the vacuum port. This resulted in the part weighing twice as much as expected which is unacceptable as this is a very weight critical application.

What are we missing here ? this also happened with another part that was around 4 times bigger and we had the same issue. We did vacuum tests and by the time we opened the part the bag was very tight so we don't think it was a leak issue. We are using peel ply between the bag and the laminate at each side and a layer of infusion mesh on the top part of the laminate.

You're missing the resin uptake on the foam core.
I'd estimate that specific core to be around 500g m2 per side. Or 1kg m2 total.

200gsm  carbon = 135g m2 /layer of resin
pvc core with infusion groove and holes 1kg m2 of resin
mesh 350-700g m2 of resin depending on type
hose  50-100g per metre depending on diameter
plus a little left in the pot!