All units received but not all units are Prime eligible

I sent my 1st FBA shipment (300 units) about 2 months ago, after 5 weeks of receiving, Amazon finally received all the units (300 units are saleable) but only 296 units are Prime eligible. It's strange.

Initially I thought it was because the receiving process was not finished yet but now it's 3 weeks after the shipment was closed, and I am still seeing only 296 units being Prime eligible.

I have called and emailed the Support team many times and surprisingly, I was told different responses every time, and sometimes the support team kept saying "it's weird", but I really need an answer.

Anyone experienced something similar before? Any advice is appreciated, thank you!