Recovery VAT from EU Country

Hello expats!

One question about VAT rules. I am an independent consultant registered and based in Belgium. I have invoiced a client based in the Netherlands and included the standard 21% VAT. After they paid, they are now coming back to me saying they were not able to recover VAT from the Belgian administration and are requesting me a refund of the 21% VAT they had to pay on my invoice. Are they right to claim that and, if I need to refund them the VAT, will I be able to recover it myself from the Belgian administration (how?).

Thanks very much for any replies.