You could either be running out of memory, or specifying invalid bitmap dimensions (i.e. w=0, h=0). You can check your current memory usage by running your app, and telnetting to the roku on port 8080. Once connected, run the 'r2d2_bitmaps' command to view video memory usage.
Also, it's worth noting that bitmaps are not de-allocated until the *next* call to SwapBuffers(). This means even if the variable which references the bitmap has gone out of scope, the memory may still be in use.