In the game sacra terra I can not use the syringe to pick up the stone acid despite the hint guiding me to do so.

That would be a question best taken up with the game developer or in a forum dedicated to that game, if one exists. It could well be a bug in the game. You can try restarting the iPad, though, and see if that help:

http://support.apple.com/kb/HT1430

Regards.

You really need to take this up with Big Fish Games first (assuming I found the right "Sacra Terra" game), not the iTunes Store. It's the developer's responsibility to address use questions and fix bugs.

http://bigfishgames.custhelp.com/app/answers/detail/a_id/362

You can also ask in their own forums:

http://forums.bigfishgames.com/forums/list.page

If you can't get any help from Big Fish, you can try reporting the problem to the iTunes Store here:

http://www.apple.com/emea/support/itunes/contact.html

but all they would be able to do is provide a refund assuming you paid to unlock the full game. Whether they would do so I don't know, but you can ask. Contact the developer first, though, and then if you get no help from them you can at least tell the iTunes Store that you tried.

Regards.