Not sure whether this is AE, but that's how I would say it. "Trying too hard" already implies that the result is not great, so you don't really need to add "but it just didn't work".

I think this might fit!

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Note added at 29 mins (2011-06-12 09:35:10 GMT)
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OR:
-close, but no cigar (how I know the expression, Nicole)
-better luck next time
-we try harder, you should, too (possibly passé)
-should have left it to those who can/know how
-better luck next time

or, nicely:
-(At least) they tried
-much effort, but little result

Hope I could help a bit! Have a lovely Pentecost - or enjoy the free time!

couple of three more...

The opposition of "wollen" und "können" is vaguely proverbial (for instance referring to art), so maybe someone can come up with a more idiomatic phrase in this vein?

says much the same thing IMO, in this context. I'd almost risk a CL4.

maker thought the product was good enough to bring to market, customer thinks otherwise.

would also be a normal colloquial reply on such a q'naire.

Somewhat prosaic but gets the meaning, I think.

This "wie" at the beginning is very strange. Possibly a misheard "viel"? "Viel gewollt und nichts gekonnt" is a familiar expression, the meaning is clear enough. "Wie" is weird in this context. If you can't clarify this with the client I'd suggest just going ahead with "they tried but failed": the German isn't idiomatic, either.

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Note added at 1 day5 hrs (2011-06-13 14:56:38 GMT)
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Shame on me! It really might be regional. Thank you, Nicole.

A late offering, just in case there is an outside chance that this is what is meant by the respondent. The emphasis may be a little different from most of the other suggestions, but it is a similar sentiment - it may appear to be a good product, but does not actually live up to expectations or to the claims/promises made. This expression and its variants all show, but no go or all show no go, used originally with reference to cars, do all crop up a lot these days with reference to mobile phones (style over substance, etc.), e.g.

All show, no go!
Pros: Flashy design, lots of options. Excellent reception.
Cons: Poor battery life, poor software, poor performance.
The Bottom Line: Buy this phone if all you care about is looks. If you want a quality, functional phone, look elsewhere.
http://www99.epinions.com/review/MOTOROLA_UNVEILS_MOBILE_PHO...

The contributors to this Leo discussion consider this a possible approach:
Das sieht doch aus wie gewollt und nicht gekonnt.
Eine spöttelnde Redewendung für etwas auf den ersten Blick optisch Ansprechendes, das sich aber bei näherem Hinsehen als unpraktisch, unbrauchbar, etc. herausstellt. Derjenige, der das sagt, regt sich darüber auf, das die Sache nichts taugt.
Translation - all show and no go
http://dict.leo.org/forum/viewUnsolvedquery.php?idThread=179...