“Judi Dench is the latest actor to criticise younger performers for poor diction.
In a speech before unveiling a blue plaque at the former home of actor John Gielgud, Dench complained that younger actors are not applying themselves to develop their vocal technique, expressing her frustration at not being able to hear them properly.” The Stage

There are lots of reasons for poor diction among some young actors.
Not always because they haven’t been to Drama School.
With the casting done by sending self-tapes, the emphasis is on the look and instant openness to the camera.  Providing the actor says the lines with conviction and is entirely convincing in the role a director will be satisfied that they can direct them and they’ll be good in the roles. Often they are, but for more demanding parts – vocal technique is essential.
Actors like to deliver in their “natural” voice and are keen to preserve this.  When younger actors come to me for voice coaching they are more concerned with learning accents than with learning vocal technique. Depending on the job, I’ll coach vocal technique along with the accent. Maxine Peake has a fantastic vocal technique and her ability to stay with her own accent as well as embrace other accents is a great role model for young actors.
The most successful actors have been fortunate to realise early on that their voice is very much part of their craft.  When an actor is working all the time then the voice grows and technique is passed on by the more experienced actors.  When you’re not working all the time – actors need to build in a daily routine of simple voice exercises that they can incorporate into their lifestyle.  Love of poetry and reading out loud along with the daily voice exercises will always support them – mentally as well as vocally.