There can be different reasons for that:
- Do you want to resize a machine? Then may you have used more resources (e.g. disks) than supported on A2
- Do you have a special subscription (Azure Pass, MSDN etc)? Then different limitations apply.
- Do you want to create a new machine? Then you should see at least a grayed out symbol if you select to see all sizing options, and not only the recommended ones.
Can you give more details?