The noun flopbuster denotes a film which fails to achieve the commercial success that was expected.

This noun is from flop, denoting a failure, and -buster in blockbuster, denoting a film which achieves great commercial success.
—Cf. the noun bonkbuster, from bonk and -buster in blockbuster.

The noun flopbuster occurs, for example, in the following by Nicholas Barber, published on the BBC website on Thursday 6^th July 2023:

Does Indiana Jones and the Dial of Destiny mark the era of the ‘flopbuster’?

Indiana Jones might be able to unearth the Ark of the Covenant, but can he fill cinemas? Apparently not. Fifteen years on from Indiana Jones and the Kingdom of the Crystal Skull, Harrison Ford’s whip-cracking archaeologist is back in Indiana Jones and the Dial of Destiny, but, so far, audiences haven’t come back with him. The film made $130m (£102m) at the global box office on its opening weekend, which may sound impressive, but which is actually “underwhelming”, Rebecca Rubin wrote in Variety. This is, she pointed out, “one of the most expensive movies ever, [costing] $295m before marketing”, so it is unlikely to make a profit in cinemas.

The noun flopbuster has recently been borrowed into French. The following, for example, is from Hollywood : le spectre de l’ère du “flopbuster” [i.e., Hollywood: the spectre of the “flopbuster” era], by Jean-Samuel Kriegk, published in Blast : Le souffle de l’info on Monday 17^th July 2023:

Certains observateurs américains estiment qu’Hollywood serait rentré dans l’ère du “flopbuster”, un concept associant flop et blockbuster, ce dernier terme désignant les films à gros budget.
     translation:
Some American observers consider that Hollywood may have entered the “flopbuster” era, a concept associating flop and blockbuster, the latter term designating the big-budget films.

The earliest occurrences of the noun flopbuster that I have found are as follows, in chronological order:

1-: From the column In my view, by Jim Keay, published in the Evening Post (Bristol, Avon, England) of Friday 3^rd January 1986:

Mini-series a flopbuster

JUST two days gone in 1986 and already we have a contender for worst mini-series of the year, a category in which there is always intense competition.
According to the ecstatic voice at the end of last night’s second part, “three generations of forbidden passion collide” in tomorrow night’s climax to Mistral’s Daughter (HTV).
I only hope the collision isn’t too much for the ageing actresses in masks of make-up who have been forced to utter such ludicrous lines as “I think we should start a family” (Lee Remick, who won’t see her half-century again).
[…]
It’s not just the women who are ze feesh out of ze watair (French accents are attempted by several members of the cast). Keach, wearing a red wig and intense expression, doesn’t bother with an accent and plays the painter Mistral as Sylvester Stallone would.
And what are our own admirable British professionals such as Joanna Lumley, Victor Spinetti, Ian Richardson and Angela Thorne doing in this cliche-clogged tale? Earning good money, I suppose.
I wish they didn’t make them like this any more.

2-: From Puttnam may be another story in the era of Conglomerate Hollywood, by Jack Mathews, published in the Las Vegas Review-Journal (Las Vegas, Nevada, USA) of Monday 14^th September 1987:

Neither Tri-Star nor Columbia is having great years. Tri-Star has had one medium hit in “Blind Date,” Columbia has connected with “Roxanne” and “La Bamba.” “Rambo III,” the one film that figured to be a sure hit for Tri-Star, had script problems and did not get made. “Ishtar,” which was a $40 million flopbuster for Columbia, was the responsibility of a previous administration.

3-: From ITV sailing into blue waters!, published in the Herald Express (Torquay, Devon, England) of Saturday 21^st October 1989—War and Remembrance is a U.S. television miniseries:

ITV’s arts flagship “The South Bank Show” has stepped into a new controversy […] because of swear words contained in a profile of author, actor and playwright Steven Berkoff to be screened on Sunday, at 10.35 p.m.
[…]
Berkoff, who appeared as Hitler in the flopbuster “War and Remembrance” earlier this year, makes no excuse for his abrasiveness.

4-: From the review of Men in Black (MIB), a 1997 U.S. cinema film—review by Sarah Reedman, published in the Fenland Citizen (Wisbech, Cambridgeshire, England) of Wednesday 30^th July 1997:

FLOP-BUSTER!

Every now and again a film is produced which promises the Earth but delivers very little, and, sadly, the so-called summer blockbuster, MIB, is one of them.

5-: From The Age (Melbourne, Victoria, Australia) of Sunday 28^th February 1999:

Flopbusters

They may be critically acclaimed overseas, but local films just can’t draw the locals. Lawrie Zion examines a trend that sees us preferring American mediocre to home-grown cutting edge.