The Screw Stopper problem

During WW II, such a little thing as a screw stopper could impact the ability to produce bottled beer. The shortage of them worsened as the war progressed. On 17th August 1943, Barclay Perkins sent this letter to all their tenants:

“re: Screw Stoppers for Pints and Quarts
The shortage of Screw Stoppers is now becoming acute. If we cannot "get back" the number of Stoppers we send out there must inevitably be a further reduction in the output of Bottled Beer. Will you therefore, please give us your help and endeavour to see that as many Stoppers as possible are returned to us with the empty bottles as quickly as can be managed.

Should you have any surplus Stoppers please telephone Hop 7171 and notify our Order Office. Arrangements will then be made for collection.”
Barclay Perkins Circular Letters held at the London Metropolitan Archives, document number ACC/2305/01/521/1.

That letter doesn’t seem to have helped much, because just a few months later, on 17th April 1944, there was a further correspondence from the brewery:

“re: SCREW STOPPERS
You may remember that I wrote to you on August 17th, 1943, pointing out that the shortage of Screw Stoppers was then becoming acute. The position is now more serious than it was then, and owing to the fact that we are unable to replace Stoppers which are lost or not returned to us, it may become necessary to make a cut in the supplies of Screw Stoppered Bottled Beers, to as much as 30% of the normal quota if we cannot get back more Stoppers from our customers.

Will you please give us your help as you did before and make every endeavour to see that Bottles are returned to us with Stoppers, and if you have any surplus of Stoppers on your premises kindly telephone our Order Office HOP 7171 and arrangements will be made for them to be collected.”
Barclay Perkins Circular Letters held at the London Metropolitan Archives, document number ACC/2305/01/521/1.

Why were new screw stoppers unobtainable? I assume because they contained rubber, which was needed for war purposes. Crown corks required none, being constructed of metal and cork.