The problem you are experiencing is due to the fact that you have two different types of partitions; primary and secondary. The numbers 1 to 4 are primary partitions. Five and up are secondary. The secondary partitions rest on top of the primary partitions, so you may not be able to add any more secondary partitions without adding another primary - IE 3 or 4. So to use up all the space on your disk, you have to add another primary partition.
You may find using fdisk, which comes with some Linux distros like RedHat, much better for doing complicated partitioning tasks. You can work directly on the disk, instead of hoping the automatic partitoning tool has done it right. The control is greater, but the risk is higher too, so don't say I didn't warn you.
Also, I think you can only put four secondary partitions on each primary, and I'm unsure if it's wise to go above eight partitions in total, but this is beyond my limited knowledge of things.
Hope this helps,