I have connected two notes using bootnode in ethereum private blockchain but mining is not started why?

Question

I have generated the enode from Bootnode in Ethereum private network and i connected three individual nodes using that enode through Bootnode, and i have checked whether that nodes are connected or not with my current node using the command "admin.peers". Screeshot1

those three nodes are conected successfully

then i start the mining using the command miner.start() Geth not started Screenshot2

As of my observations i am getting the line constantly,

Line => "Regenerated local transaction journal transactions=0 accounts=0" for every one hour as we can see in the above screenshot But even single block is not created

Question : Why is mining not started, How to start mining? please give me some suggestions and guide me if i am wrong

I have created simple gensis.json file. Please find below

{
"config": {
    "chainId": 1015
},
"difficulty": "0x400",
"gasLimit": "0x8000000",
"alloc": {
    "ef2df6e0a312875ca7e35d8d3579cf66df558193": {
        "balance": "30000000000000000000000000000"
    }}}