You really don't lose value by getting any certain number of votes (afaik). What happens is that you get most your votes early on after a post is made (say the first 6 hours or so.) After that, there isn't much activity. During this 'slower' period people many other votes are still going out. This causes vote dilution...since how much the payout is, is really a proportion of the daily reward pool versus a fixed dollar amount.
"As more votes go out (not on your post) it shrinks the proportion of votes on your post compared to 'all votes cast'.
There are also other effects on the payout, such as the 3.5 day average prince of steem which when viewed in dollars, is relative to the price of bitcoin.
Thanks. That helps. I've heard that if you just go around liking heaps of posts then your vote becomes worth less. Is that also a thing?
Exactly. Your vote power effects how much you give with a vote. For example if your full vote gives $1 at 100% vote power, it would give 50 cents at 50% vote power.