There are several things to consider when preparing a poster for print. They are briefly mentioned in some tips here
There is a more in depth guide to poster printing here
And a good step by step guide here
To briefly cover it though. You need to make sure that your image size and resolution are appropriate for the poster that you make. You don't want to have to make the picture larger after you've designed it. It's better to make it larger from the beginning.
You can check your current image size by going to Image > Image Size in Photoshop or by hitting CTRL+AlT+I
If the image size here is not the size that you want to print then it would be best to open a new file with the appropriate size preferences. (File > New or CTRL + N) In these preferences you want to ensure that the width and height are set as you need them (which could be a little larger than the final poster size if you want to accommodate for a bleed and crop marks) You also want to make sure that the Resolution is set to at least 300dpi to ensure that your image doesn't become pixelated.
When printing it's also better to set your colour mode to CMYK instead of RGB which would be more appropriate for web.
There are lots of print resolution questions here that you might like to consider reading over to help with your current issue.
Also don't just change your resolution on the image size of the poster that you have. If you are increasing this (i.e 72 dpi to 300dpi) then your image will become significantly smaller to preserve quality and you'll just run into many more issues.
If you need to significantly increase the size of your file you are better off in terms of quality to just remake it using your current layers for reference in the new file.
I hope my advice and links help you out. Glad to see you managed to recreate the background that you were having issues with. :)