If you replaced the bar AND the chain, you'll need to make sure the chain is the correct chain for the particular bar you've used. If you're only replacing the chain, you need to make sure the chain has the same # of drive links as your old chain (maybe one or two more, if you've had some removed because of chain stretch).
On that particular model, for a 20" chain, you could have a 78 OR 80 drive link chain (drive links being the part of the chain that rides in the groove of the bar). The correct drive link count is determined by the bar you've installed and should have been made clear by whoever you purchased the bar from. Some brands use an 80 drive link chain, but MOST use the 78 drive link chain.
If the bar you purchased uses the 78 drive link chain, but you have an 80 drive link chain, then no matter how far out you adjust the bar, you may not get the chain tight enough. PartsBuggy shows a listing of which bars use the 78 drive link chains here