If you look at https://www.bloomberg.com/graphics/covid-vaccine-tracker-glo... and scroll down to "U.S. Vaccination Campaign" you will see that in the US 74.5% of the vaccine doses that have been delivered have actually been used.
This could happen for a combination of two reasons, as far as I can see: either delivery is ramping up so fast that most of the delivered doses were delivered very recently, or because people are doing a terrible job of using doses that get delivered.
My impression is that the latter is a larger factor than the former, so far, based on the wide variation by state in the "Supply used" column. West Virginia has used ~96% of their supply. D.C. and Rhode Island have used only 60% (and it's not like their populations are larger or more spread out than West Virginia's!). And I should note that I can imagine various poverty-and-social-whatever issues D.C. might be facing, but they don't obviously apply to Rhode Island in the same way.
Things like http://www.arnoldkling.com/blog/virus-update-3/ item 3 are pretty common: crazy vaccine-distribution setups meaning people who want vaccines can't get signed up to get them, while vaccines are getting thrown out due to spoilage.
Things like https://www.nytimes.com/2021/02/10/us/houston-doctor-fired-c... are, I hope, not common, but you can bet that case will have a chilling effect on efforts by doctors to avoid wasting vaccines.
https://www.theguardian.com/global-development/2021/feb/14/w... includes this incredible from a major vaccine manufacturer: "Instead we have a patchwork of approvals and I have 70m doses that I can’t ship because they have been purchased but not approved. They have a shelf life of six months; these expire in April."
Does that sound like a manufacturing supply constraint to you?
Similar is https://marginalrevolution.com/marginalrevolution/2021/01/ap... where we have a vaccine, and a factory that can produce it, and the vaccine is approved in the UK, but it's not approved in the US. This is also not a supply constraint.