As a non resident they would have kept 3% of the selling price to pay for outstanding taxes such as capital gains.
If the small sum you received back was from the tax authorities then I assume the rest was kept for those taxes.
You may not have made a gain on the price but they can also get you for the gain on the price of the land called the plus valia tax. Perhaps the money you gave the solicitors went to pay for that.
It is also difficult to get money back from the tax people if you have changed or closed bank accounts since selling. They will make the payment to the account they have listed for you but, if not accepted, they will not try again.
Bit awkward if the solicitors won't reply to you, though.