Actually they did strike cents in 1815. A soon as the war ended the mint ordered planchets from Boulton and the arrived in December of 1815. During the last week of the year th mint began striking cents again but used 1816 dies. So there were no 1815 cents, but cents were struck in 1815. No a shortage of bullion was not a problem. In the early 1930's the Treasury had hundreds of millions of oz of silver on hand and was about to embark on a major silver buying program that caused the Treasury stockpile to swell to over two billion oz by 1935. And as for 1921, the mint had plenty of silver on hand for the minor coins because none of their stock was going into the 1921 dollars. Under the provisions of the Pitman Act all the silver for the dollars had to come from newly mined silver purchased on the open market. The problem in 21 was a lack of coining capacity as everything was tied up striking silver dollars.
Which brings us to an interesting question: When was the last year that a mint failed to produce an entire complement of denominations?