Why do people ignore me when I talk in a group?

Usually nobody means to. A group decides quickly, and mostly without words, whose comments it follows. If the group hasn’t yet placed you among the people it listens to, a good point can pass by as if it was never said.

What’s usually going on

In any group there is an unspoken order of attention. People glance at whoever they expect to speak next, pick up the thread from whoever they already trust, and let other comments drop. That order forms fast, often in the first few conversations, and then repeats itself.

Timing matters as much as content. A point made after the conversation has moved on, made quietly, or made as a question can be heard as background. Then someone else says something similar a minute later and the group responds to them.

Being new makes all of this stronger. Until people have a sense of you, they default to the voices they know.

What you can do

Speak earlier. The first few minutes of a conversation are when the group is still deciding whose thread to follow.
Connect your point to the last thing said (“Building on what Sam said…”). It gives people a reason to turn toward you.
Make one clear point, then stop. A single sentence lands better than a careful paragraph.
Follow up one to one. A quick conversation afterward builds the relationship that makes the group listen next time.
Watch who does get heard, and when. The pattern tells you more than any one conversation.