Hi hhrishi ,
Did you do the following?
Select the Armature, go into Pose Mode, SHIFT+Select the Tube and then switch to Weight Paint Mode.
Also make sure that you have Parented correctly and that Edit > Lock Object Modes is disabled.
That's all I can think of at the moment (but I am not a Rigger).
Hi hhrishi ,
When you have a Bone chain, none of the Children can be Moved in Pose Mode:

Only the great-granddad can Move the other Bones can only Rotate.
So it has nothing to do with being in Weight Paint Mode.