Back in the day, stiff links on newly closed chains was very common. Some chain tools had a step made expressly for increasing the gap between the outer and inner link. Without the special chain tool, you can usually free the link by putting your thumbs on the suspect pin and flexing the chain sideways (the way chains don't flex). This makes the inner link wedge the outer plates apart, so do it slowly and by degrees only until the chain is free.


BUT-- and this could be very important and spare you some grief. so read below before riding

Modern chains, anything marked 8s or more on the package, cannot be properly closed by pushing a pin back in. The pins on these chains are peened over the outside of the plate (like the rivets in bridge construction) so the plate cannot be pushed over the end of the pin. This is necessary because the chains were made narrower by using shorter pins that don't stick out as far as they used to.

So, while you might free the link, if this is a flush pin chain, you broke the head off the pin when you pushed it out, and it won't grow back. That means that the plate isn't secure and may (probably will) end up spreading beyond the end of the pin causing the chain to snap under load.