The problem with remaking Dads Army for Cinema is the same one that faced all adaptations of TV sitcom from half an hour to an Hour and a half.
The cast of the new dads army film do a good job, they are not doing impression of the original cast but putting their own interpretations on the roles.
The production value are high and the plot fine.
As the reviews have made clear, what is lacking is enough jokes. This I think is essentially the problem of having just one person writing the script.
Mark Kermode has a very perceptive review here: