When mining Ethereum, a large amount of data called a DAG must be loaded to the GPU memory. This DAG is increasing with time as more and more Ethereum blocks are being mined. For this reason, GPU's with only 3 GB GPU memory haven't been able to do Ethereum mining for a while and now are getting to the point where 4 GB GPU memory cannot be used either. To be able to mine Ethereum, it's recommended to have 6 GB GPU memory or more.
To confirm if the mining problems are related to the DAG size, please start the miner with the Diagnostics button to see the reason why the Ethereum mining failed.