texts don't downroad

I've had my pixel 2 for 6 months. I've never seen this issue before in any of my other cell phones: sometimes ill get a text that will come on to my cell but the message wont download. it'll have a bubble with a 'download' to click on, then it downloads. it doesn't seem to be related to wifi, wifi signal strengh. it seems to happen randomly and i cant figure out a pattern as to when and/or why it happens. Any help out there?
thanks.

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