i am not able to checkout cad part, as i was doing earlier. All drawings & parts are in inwork state & it shows user don't have necessary permission
what must be the reason?
Probably issue related to access control of User.
You can go to Object Details page --> Actions --> Manage Security. In the Access Drop down menu select My Access. System will show access details.
I hope this will solve your problem.
In Addition to this MS log will have more details so need to look into it.
Only use the embedded browser, don't use a stand-alone browser. (you are bound to make an error if you do)
Update and Synchronize your workspace. Then verify the State is what you think it is.
If that doesn't work, as Sergey says, post your method server log for the time period.
Could be because of the access issue , also depending on the state of the cad document , Authors can not checkout is object is not in creation state