Your bottom layer isn't squishing enough so the bottom layer appears underextruded and isn't sticking well.
You need to move the nozzle closer to the print bed. If this is an Ultimaker then turn the 3 leveling screws about 1/2 turn CCW to move the bed up. Turn off any auto leveling if you have a UM3. When the first layer is going down watch it carefully and adjust the 3 screws by equal amounts (assuming you leveled the bed) until you have the bottom layer squished nicely. here's a video with probably much more detail than you want but it will make you a "make my part always stick" expert:
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gr5 2,295
Could you post a photo please? It usually takes several layers to go from infilled layers to solid layers because the bridging isn't perfect but 1.2mm should be enough.
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