Ancestry DNA matches without common ancestors

[danalana]

Just wondering how to go about unravelling where the "links" are (when a common ancestor isn't named) with Ancestry DNA results.
I have over 15,000 matches who are "5th-8th cousins" but, being a novice at all this, I have absolutely no clue where you would even START in trying to find out when the links come from.
Disappointing on my results (despite having nearly 200 matches with 4th cousins or closer, only a tiny number of them have "common ancestors" listed for me to view via the "thru lines" feature).
Any tips?

You're probably going to need to go back as far as the 1700's to find a common ancestor then.

I haven't done an Ancestry test so don't know how it works, but if you assume that on average each generation adds 30 years, that's where it would take you.

CA
Children
1st cousins
2nd
3rd
4th
5th
6th
7th
8th

Assuming that the 8th cousins were born around 30 years ago (for clarity) that would make the common ancestor born roughly 300 years ago. With a gap of 25 years between generations, the common ancestor of 8th cousins would have been born roughly 250 years ago.

I look at them one by one to see if there are any common surnames, and I have identified a few connections that way. Also, you can look at their surnames that are not in common with yours to perhaps see if there are alternative spellings to your names.

Common locations might also help.

Checking the shared matches can point you in the right direction too, because if you know what branch of your family they are from you at least know where to start digging.

Also, get to know all the other surnames in your extended family, i.e. not in the direct line, because even if a match hasn't researched as far back as the common ancestor, you may still spot a familiar surname in their tree that helps you.

I find a lot of my matches have private trees so I can't view the their tree, or they have a very small tree with only a few people in it. Others don't link their tree to their DNA test at all. Others might do the DNA test to see their ancestry make up but they aren't interested in tracing relations.