Lots of actors lost out during World War II. They went off to war for 6 years while other actors stayed home and acted in movies and plays and kept in the public eye. The soldier-actor comes home...and if he was a "juvenile" lead, all of a sudden he's too old to play that lead...so he gets relegated to character roles.
Especially if he's 5 ft 7 or less, which I suspect is what Lack was.
The Times, Tuesday, Aug 12, 1980
Mr. Simon Lack, the actor, who died on August 6, had a steady career that began, appropriately enough for a Scot, in the Brandon-Thomas repertory companies at Edinburgh and Glasgow during the season of 1935-37. Born at Cleland on December 19, 1917, son of Alexander and Euphemia Macalpine, he was educated at Eastland Academy, Glasgow.
Reasonably, too, his first West End part, confidently managed, was also a Scot, the Glasgow financier’s son, part of a slight love interest, in Bridie’s The Last Trump (Duke of York’s, 1938). In the following year he had the brief and poignant scene of the dead airman, reappearing in his mother’s memory, in [J.B.]Priestley’s Music at Night (Westminster). Later for six years he was out of the theatre, serving with The Lancashire Fusiliers and The Buffs (and being mentioned in dispatches.)
Reappearing on the stage in 1946 [at age 29], he found an odd variety of of plays and parts: David in H.P. Templeton’s You Won’t Need the Halo (Arts), set in a London mission-hall; another David in Peter Yates’ The Assassin, a bold attempt at a verse tragedy in the grand manner (Lyric, Hammersmith); Gilbert in Jane, S. N. Behrman’s version of a story by [Somerset] Maugham (Aldwych, 1947); and Sextus in Cage Me A Peacock, a musical (Strand, 1947) set in classical Rome.
He had a long run as an amiably sympathetic young man in [Noel] Coward’s Relative Values (Savoy, 1951); and at various times played one of the undergraduates in Charley’s Aunt, and the elder son in Enid Bagnold’s The Chinese Prime Minister (Globe, 1965), besides his work out of London and in films.
He also made a number of television appearances and had been seen recently in Enemy at the Door and Telford’s Change. He had a part, too, in the new BBC television production The Borgias.