The 2019 Ram 1500 is experiencing a rather bumpy launch as EPA delays its approval for key versions of the redesigned pickup truck.

It’s been nearly five months since the 2019 Ram 1500 -codenamed DT- entered production but so far only two of its variants have received certification by the EPA: the rear- and four-wheel-drive versions fitted with the standard 5.7-liter V8, Automotive News reports.

The first examples of the DT Ram 1500 began arriving at dealerships in the second half of March but dealers still wait for the 3.6-liter V6 model and the mild-hybrid 5.7-liter V8 equipped with the optional 48-volt belt-start generator. V6 models are fitted with the belt-start generator as standard.

“I have customers looking for them and asking about them every week,” said one Ram dealer in Michigan on condition of anonymity.

There’s no official reason why the mild hybrid versions of the 2019 Ram 1500 are still not available. FCA has declined to comment on the matter, but company engineers could reportedly be working to fine-tune the powertrain before releasing the models.

After all, FCA has a troubled past with models being rushed into the market, with the example of the 2014 Jeep Cherokee and its unfinished nine-speed automatic transmission springing to mind.

Another reason for the delay could be the EPA being much more stringent with car makers in the wake of the VW Dieselgate scandal. FCA and EPA are still battling in court over the previous use of the 3.0-liter EcoDiesel powerplant in the Ram 1500 and Jeep Grand Cherokee.

The slow rollout of the 2019 Ram 1500 appears to be suppressing the pickup’s sales, with analysts saying that the lack of a V6 model this long after the launch is a problem. Ram pickup sales were up 4.3 percent in May compared to 2017 but it was the first month of the year with a positive outcome. Year to date sales are down 8.4 percent, with FCA US saying that retail sales of the Ram 1500 pickup were up 18 percent in May.

Ram said last year that it plans to knock the Chevrolet Silverado out of its No.2 spot in the full-size pickup segment in 2018 by continuing building the previous-gen Ram 1500, backed by big discounts, while ramping up the production of the new generation.