If you want something to sit in the mix, you need to process it so there are common sonic elements like running the vox into a reverb that is being used for something else. Group/effect channels work good for this. The EQ of the vox may also be standing it out. If there are guitars, boost some high end to them to help mask the voice or cut some of the high freqs of the voice.
Lots of ways… Pull down the vox fader. Compress the heck out of it, dump it into a reverb… that will send it to the back. Narrow it… pan it off centre.
You can also work on groups of instruments and make them sound denser using a compressor. If you compress electric guitars for instance, compressing kinda hard, it will create a larger “wall of sound” to sit with the vocals.
The BEST thing you can do is to determine why the vocals are not sitting in the mix. If you figure that out, you can make them sit. Note the qualities ogf the mix without the vocals and note the qualities of the vocals alone. Write things down and learn why.
All techniques to achieve something are based on the entire mix and that there will be the guide to determine what needs to be done to achieve a certain thing. Ambient music requires different things than punk rock than acoustic folk than rap than etc… see what I mean?