we know that they are both off to slow starts so I went to fangraphs to look a little deeper into the numbers.
Dunn: The one thing the jumped out was BABIP which was very much below norm(.243 vs .292 career) ... his LD% is down slightly(17.8). The other difference is his GB/FB ratio is 1.0 versus 0.73. Obviously less FBs generally means less HRs. His contact percentages show nothing major going on. Too many GBs, some bad luck and a bit of a slow start seem to be the only issue.
Jr: While his BABIP is down(.265 vs .285 career) his LD % has dropped considerably over the last 3 seasons and is down to 12.5% this year. His HR/FB % is down more than 50% from his norm(8% vs 17%). In Jrs case it could be a slow start but it also could be father time catching up causing a serious drop in power. When you reach the late 30s and aren't juicing there is always the distinct possibly that the skills have simply disappeared. Jrs numbers will need to be watched closely to see what is happening is an aberration or a new trend.