Unable to mine on testnet
One possible reason may that with Bitcoin Core 0.13.1, it was mandatory to use SegWit-compatible mining software. When SegWit transactions are included in a block, a coinbase commitment to the witnesses is required by consensus rules. As the eventual selection of transaction in the block is done by the mining software, old mining software cannot be used to construct a SegWit block. On testnet, SegWit (BIP141) has been active since May 2016.

In Bitcoin Core 0.14, a change was made that permits old mining software to continue to operate. When such software is detected, Bitcoin Core will simply return a block template that excludes any SegWit transactions, alleviating the need for a coinbase commitment.