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?
If you are talking coins struck for circulation, there haven't been dollar coins struck since 2011. The NA dollars are still authorized as circulation issues.
Also no Standing Liberty quarters minted in 1931. (There were no quarters, half dollars, or dollars dated 1975, but they were minted in 1975; they just happen to have the 1776-1976 date and bicentennial designs that were minted from 1975 to 1976.) Walking Liberty half dollars have more gaps than that: none were minted in 1922, 1924-1926, 1930-1932. I put together a set for my grandfather, born in 1924, and there was no half dollar I could put in there. (Years after he passed away and the set was given back to me, I got the 1924 Huguenot commemorative to put in that half dollar slot, the only US half dollar minted in 1924. The empty space was bothering me!) Longest gap in any US coin design was with the Susan B. Anthony dollar, that had none minted from 1982-1998 until they were minted again for the final time in 1999. (This is if you don't count bringing back the Morgan and Peace dollars, or the gold versions of the Mercury dime, Standing Liberty quarter, and Walking Liberty half dollar. But 1999 SBA dollars were actually intended to circulate!)
In the early 1930s the economy was on its back because of the Great Depression. There was no demand for new coins.
1932 and 1933 were the depths of the Great Depression. At that point the gross domestic products of this country had contracted 40%. Nothing else has even come close to that. On the eve of Pearl Harbor, in 1941, unemployment was still 14%, and that was eight to nine years later. Demand collapsed, jobs collapsed, everything collapsed. There was no need for coinage in 1932 or 1933. The Lincoln Cents minted then sat in vaults until the slow pace of recovery got moving by 1935. What cured the Depression? The Second World War. In December 1941 unemployment was 14%, and by May 1942 it was 2%.