Networks

This might be a machine account problem, or a DNS problem, or a permissions problem, or a network configuration problem.

Need better and more detailed information.

Most likely there is an issue on the individual computers. Still need more information.

What server is being run? What are the OS' on the machines? What happens when you try to map to the network drives? Are you using login scripts?

Take a look at the netstacks on the computers that don't work and compare them to the ones that do. What is different?

Are you using the "System Login Script" for everybody or have you set up user login scripts. I recommend using only the "System Login Script" found in console One to avoid a lot of problems.

There are several things to check. THe version of NetWare and a sample of the script would help. Is it a container level script?? Or is it a User level script? Or something in between?
First you should try if the users can login from another PC that does work to eliminate the possibility of a PC specific issue. Try it for all of them and note whether or not you are successful. If it works from another PC it is a problem with the PC. Are they ghosted PC's from a properly SYSPREP made image?? These are the type of things to explore.
If not able to login then you should try checking the script permissions to see if they have the ability to read the login script or the conatiner where the script is. Also check that the users have permission to the objects you are trying to map. Are they in the right groups and are the permissions present?
Good Luck and try to investigate things more. You should know this information and have it at your fingertips. Troubleshooting skills 101

Simply check the duplex speed for your nic. Most computers are set to auto-detect. Even though your switch may also be on auto-detect it may not always be the best setting for that computer. I find that 100/half-duplex works well. Play around with that. It will allow your computer to access data faster.