HI Ashikul,
1) Check number ranges for asset & Equipment.
2) As I mentioned in my very first comment. Equipment creation will create asset based on your configuration settings. Now, If your sub-equipment categories are not falling under your customization settings, then sub-equipment ( Which is nothing but an equipment only) will not generate an asset number. If you want to have one to one mapping then, maintain your config settings for all relevant equipment category & object types.
Regards,
Amol