If you have four different RAM Disk products installed and running at the same time, it's not really a great idea. This may result in unpredictable behaviour.
That aside, most likely the error message really means 'there is no single chunk of RAM large enough'. You may have 2600 MB free, but the largest continuous memory block is 260 MB or less due to memory fragmentation.
A restart may help, though it may as well not, as multiple RAM Disk products are likely to fragment the RAM space.