Why studies are not feasible?

Why some studies are feasible whereas others not?

Obviously, main reasons of non-feasibility are as follows:

Patient recruitment—where difficulties were encountered in recruiting and/or retaining the participants during the study or low eligibility of patients
Trial design/methods—where difficulties were encountered in blinding, inappropriate inclusion/exclusion criteria, general method/design issues
Intervention—where the intervention lacked sufficient promise of efficacy/effectiveness, was not used by participants, or was not practical
Outcome measures—where outcome measures were not acceptable or could not be collected.

Nice article describe potential issues during feasibility evaluation: