RFID - Metal blocking receiver issues?

Please can anyone advise?

So we've built a prototype that detects when a bottle is opened
And triggers a software response. We need to build the real
thing next.

The prototype:
We've used a small passive UHF RFID tag mounted on the
lid of the bottle, hidden under foil. When the bottle is opened
this tag pops out and can be sensed. We have a long range
RFID scanner that recognised the appearance of the tag
and send the signal onwards. This currently works ok at a
Distance of around two meters.

Problems with the prototype:
The action of revealing the tag is not very reliable. Currently we've just
experimented with various pieces of tape and foil attached to the tag
and bottle. We need to develop something reliable that could be
applied to a couple of hundred bottles. The range is a bit shorter than we'd
hoped as well - ideally we need the range to be over 5 meters. Our reader
has the option of adding multiple aerials though which may help. And perhaps
we could have a set of readers linked

It's not necessary to use RFID if some other way of detecting when the bottle
has been opened could be used instead. The reason we chose RFID is that
the bottle doesn't need anything very large added to it. We are trying to sell
these things after all.

We really could do with someone who can help make this system more reliable
Both by helping us with improving the mechanism for revealing the tag and
increasing the RFID sensing range

What way is it "unreliable" ? Not clearly detecting when unveiled, or not clearly undetecting when covered ?

Which RFID standard are you using ?