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Eventually, the promise failed to live up to the product. Image source: Stratasys.

In the end, it's important not to fall in love with your stocks. For a long time, 3D printing manufacturer Stratasys (NASDAQ:SSYS) was one of my favorites. Last month, however, I decided it was time to part ways with it. Read below to find out why.

A great beginning
I first wrote about Stratasys in September 2011, and bought my first shares in January 2012. When I did, I specifically wrote that I favored Stratasys over rival 3D Systems (NYSE:DDD) because of its growth strategy. Unlike 3D Systems, which was able to grow at substantial rates through a flurry of acquisitions, Stratasys took a much more restrained approach.

The company made two large acquisitions/mergers after my initial purchase, but I thought those were great moves. By merging with Objet in 2012, Stratasys gained a footprint in new industries, as well as greater geographic diversity. And when it announced the acquisition of Makerbot, the consumer-facing 3D printing company was growing like gangbusters.

The stock itself had a tremendous ride up during this time.

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After I added more shares along the way, my overall position in the company more than doubled.

Then, trouble began to surface
Like all industries that balloon overnight, 3D printing stocks became a bubble. When growth started to turn sluggish at 3D Systems, investors got scared, and the entire industry began a slow move downward.

This alone was of little concern to me. I bought Stratasys because I thought it provided a groundbreaking technology, and had a smart management team in place that was growing revenue with cost restraint. Just because the stock itself experienced a temporary bubble didn't mean I would sell out -- I was in this for the long run.

More alarming details, however, surfaced in February 2014. Management pre-released very disappointing results. The main culprit was the Makerbot unit, which saw revenue growth go from almost triple digits to just 7%. The company's fifth-generation printer was simply a disaster, and Stratasys took the first (of what would eventually become multiple) goodwill writedown for its purchase of Makerbot.

At the time, I still believed in Stratasys. I argued that Makerbot represented just 16% of overall revenue, and that the company was allowed, after all, to make a mistake here or there. Just as important, I noted that its growth in printers to enterprise and industrial customers was still strong. As I said at the time, "[T]he company grew overall revenue by 62%, and organic growth clocked in at 36%."

The final straw
But this final strength -- that of the non-consumer division -- has also come crashing down. In May, the company announced that organic growth was flat year over year. In July, organic revenue actually shrank 10%. And in the most recent quarter, organic revenue wasn't even included in its release. That may be because overall revenue was down an astounding 18%.

On top of that, the company has taken massive hits for goodwill writedowns. In essence, this is a way of admitting that it has overpaid for certain acquisitions. Over the last nine months, Stratasys has announced over $1 billion in goodwill impairments. Not only that, it is reviewing to see if further impairments might be necessary.

This trend represented more than enough red flags to get me out of the stock. While some might think that now is an excellent time to buy -- and, indeed, it might be -- I simply think that any sustainable competitive advantages have dried up, as has my confidence in management to grow the company sustainably.

Brian Stoffel has no position in any stocks mentioned. The Motley Fool recommends 3D Systems and Stratasys. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.