Missing Apple Funds

Added £25 gift card balances on each of my children’s account. Tried to help them makes another purchase but says there is no funds left. Checked receipts as I get an email per purchase and each used £19.98 which means one should have had £5.02 remaining while the other should have more since she had a remaining balance for last time. I’ve checked purchase history and it does not add up with the accounted purchases. No hidden purchases either.