I assume you mean you pitched it after the wort cooled to 50 degrees?
You'll usually find the recommendation on pitching lager yeast to do so at warmer temps and then take it down to your fermentation temp. If the wort was 50 degrees and the yeast was 50 degrees, it's going to take a long time for that yeast to get going.
You can see this in the following from White Labs:
https://www.whitelabs.com/faq/brewing/what-temperature-should-i-pitch-lager-yeast
Yours will go at some point, but as they note above, it will take 48-72 hours for activity to appear.
One other consideration: how did you oxygenate your wort?
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I always pitch lager yeast about 70 degrees, wait 6+ hours at that temp, then begin ramping down to 50 degrees. That gets the yeast going before I slow it down at the lower temp.