Siri does unfortunately do that occasionally. Part of that is training her. When you have the chance, text using Siri's dictation and go ahead and spend some time fixing any errors that she makes.
Make sure your emphasizing other words, make any punctuation marks sound like an afterthought.
I'll try and convey in text how I speak with Siri:
Hey Siri, send a text message to Martin
What do you want to say to Martin?
Yeahcomma Earlier i put them in the blenderperiod
Your message says: "Yeah, earlier I put them in the blender."
Ready to send it?
Yes
I stress words around the punctuation and have spent a lot of time training her, and she still gets things wrong on occasion.
Speech to text isn't perfect, but as you work with both time and tone, both she and you will get better at the whole transcribing text thing.
If you're still having a hard time, you can head into an Apple Store and ask any associate how to use Siri for texting. Almost any of them should be able to help you with the tone needed to summon punctuation marks.
🙂