Can't add external .ipk to custom firmware (Image-Builder)

Hello everybody.
I'm trying to create a custom firmware for a GL-AR300M device. The firmware includes some more packages (from online repositories) and a ipk package (wwanHotspot). I put the wwanHotspot_1.6_all.ipk in the packages/ directory but when I run the make image command, after the Packages file has been produced, the procedure stops declaring that the wwanHotspot package is unknown.
What's the correct procedure to add a custom ipk to the image? I thought that putting it in the packages/ directory was sufficient.
TIA for your help.

That always worked for me. Until someone more knowledgeable chimes in -- have you checked permissions (and that it is a proper package, and not say an HTML page with the link to the file)? BTW, it's always better to copy-paste the exact error message or piece of the build log, sometimes there're valuable error codes in there.

Thanks for your post.
I've been in contact with the developer of the package and it turns out that the .ipk was built with wrong tar option, that's the reason the Image Builder did not recognize it.
Problem is solved now with a newly prepared .ipk file.