You say you did a "switchport trunk allowed vlan all", but did you actually put it in trunk mode (switchport mode trunk)?
Sounds like you're still in access mode in vlan 20.
interface fa0/0 - or whatever it is
switchport trunk encapsulation dot1q
switchport native vlan -- whatever you want or dont use it all and it will default to 1
switchport trunk allowed all --- or switchport trunk allowed vlan 20,30,40,1050 -- etc...
switchport mode trunk
Check your "show run interface fa0/0: --- or whatever your interface is and see if the settings are comparible.
Atleast that will help you get your trunk working on your switch.
the syntax might be different on the 2950. Havent worked with one in a loooong time. Been doing all my work on 2960 and 3700 series etc... newer stuff.