Dan Bailey may still be dealing with a lingering back problem that was hampering him earlier. That might explain why he hasn't displayed the the leg strength we are used to seeing.
Back problems don't just go away, in a lot of cases total rest is required for the muscles in your back to fully heal - and if it's a structural issue, then surgery may be required.
It seems Dallas has a lot of guys with back issues on the roster this year. T.Smith, D.Lawrence, D.Bailey, and there are a couple others I am forgetting. Hopefully back issues don't become the new "hamstring" for the Cowboys. Interestingly enough, a lot of back problems can arise from hamstring injuries, and vice versa.
Source: Am not a doctor, but have dealt with chronic back issues.
Cowboys, and surprised more teams don't do this, need to install a regimen of hot yoga for their players. All year round, stretching and core development. Sounds feminine, but could save us a lot of muscle pulls and tears.