Trying to get this back on topic...
Most reports indicate that the Hs-129 was a misery to fly, being too narrow, overweight and underpowered with marginal stability. It was also accorded wildly swinging priority through the first two years of design and production, something that is not conducive to rapid development and production.
The necessity for a dedicated twin-engine attack aircraft was seen by the Luftwaffe before the war commenced, but the HS-129 prototype didn't fly until April 1939 and it was accorded lower priority over types already in service once the war commenced. The success of the Me 110 in Poland in 1939 did nothing to help the aircraft, as many in the Luftwaffe considered it redundant. The 110 was faster, longer-ranged and could operate as a heavy fighter/night fighter.
With the fall of the Western European nations in 1940, the impetus for a Luftwaffe ground attack was lessened, as was the Hs-129s attack role with the entry into service of fighter bomber versions of the 109E and Me 110C/E. As a result, work on the eight pre-production 129A-0 aircraft was slow and they were put into a prolonged flight testing and modification programme.
The launch of the invasion of Russia, and the realisation that Germany was facing large numbers of heavily armoured tanks, revived the priority of the Hs-129 programme. The Initial Hs 129A series aircraft were equipped with 485 hp Argus engines, which made the aircraft's performance marginal.
The Henschel engineering team provided an updated design for a larger aircraft in 1941, but the Luftwaffe ruled in favour of refitting the existing design with the Gnome-Rhone 14M radial, of 700 hp. The 129B also had an updated cockpit and better propellers.
Henschel had all sorts of problems reworking their production line to accommodate the GR14M and fixing niggling problems with the aircraft. The first 129B aircraft were delivered late in 1941, but there were constant modifications needed to keep the aircraft in service.
Another drain on production were all the field kits necessary for the various weapon fit-outs: the MGFFs were replaced by MG151s. The aircraft has bomb carriers fitted to wings and fuselage. Designing the MK 101, MK 103, BK 3.7, BK5.0 and BK 7.5 weapon pods also soaked up engineering resources.
Finally there were the problems with the Henschel factory and with Gnome Rhone. The Henschel works were at Kassel were bombed heavily once in 1942, twice in 1943 and three times in 1944. The 1942 raid disrupted production and a decision was made to disperse some Henschel facilities, although I'm not sure if aircraft facilities were directly affected.
Forced labour was used at Henschel plants, meaning production quality was extremely poor. Henschel was also involved in the production of Panzer III and Tiger I tanks, which fought with the aircraft division for skilled labour.
After the initial captured supplies of GR14Ms were used up, Germany turned to new production engines. Unfortunately for the Hs 129, production at GR was infamously slow, perhaps deliberately. 14M production was just a trickle, with the French GR works producing only about a third of German needs. The engines were also remarkably finicky for a radial: problems with oil contamination, dust ingestion and overheating were common.