Please clarify what do you mean as "link disappears". There is the phemonenon that besides all your ancestors genealogically are your ancestor, but some of them are not ancestors in genetic terms. This happens because we don't get precisely 1/4 of DNA from our grandparents. So when we take a greater number of generations shared DNA with the particular great grand(several times) parents will be around zero.
@crazypieces I am sorry I still don't precisely understand you :-) You said that you were able to trace the DNA back to "8th great grandfather". I believe that you, of course, didn't take the test from him, but you were able to confirm the relationship with another person with the DNA test and check out that you both have the same "8th great grandfather". It is OK.
But "The problem is that when I look at his father's profile, DNA tag is gone" is ANOTHER problem. It may mean different things. The first idea is that that this person has the match with you above some bunching limit. And his father - lower than this limit. It is usually 7cM in the FamilyTreeDNA service. It does not mean that you are not related with these persons. It just tells us about amount of shared DNA. Now you will ask how it may happened that the son has MORE shared DNA than father. There are many reasons:
- the artifacts of matching algorithm, because it calculates half-matches. It sometimes gives false-positive results. I.e. in fact there are no such combination in your genes.
- because there is shared DNA between you and the mother of this person. And in him both small segments from his father and mother were glued together, yielding longer segment (above the bunching limit)