I've noticed that people posting in the TT several years ago often mention the ratings of problems when they played. More times than not, the ratings they state are much higher than the problems ratings are today.
I think since then, the problems have had a chance to find a more accurate rating. It is possible that back then, easy problems had higher ratings and so your score would have risen quicker when you solved them.
Or, maybe, you're just not as sharp as you used to be. 
Good topic. It does seem that almost every problem was rated about 500 points higher in 2010 than it is now. As Kleelof said, people often post screenshots of the ratings in the comments section.
So yes, the TT is getting harder, I think, meaning the problems for each rating are getting harder.
I'm certain that I've gotten better, not worse. I remember getting +15 for correct problems but now I only get 3-5 points most of the time.
over the long term, it appears that many of the tactics get lower ratings because of partial credit, ie people solve the first two moves and win points even though they don't solve the whole puzzle.
Well looks like I'm not just becoming stupid and that the problems are just lower rated, giving players a lower rating. Thanks for your responses.
The cap has been recently changed to 8, I think. It seems to have stabilised for now, but TT had been experimental with the rating a few months ago. There was a period when failing a problem would cost you a hefty -18, yet passing puzzles, even very quickly, rarely seemed to award more than 10 points. Someone who wasn't really trying during that time could have sufferred many hundred point losses and now find himself to be grinding back up very slowly.
But it's mostly what others have said about the rating of the problems now vs in the past. http://www.chess.com/forum/view/suggestions/tactics-trainer-problem-difficulty-amp-ratings
Bottom line, when it comes to the TT, a rating of X in the past does not translate to X in the present. Your current rating is only meaningful relative to its current environment and dynamics.
Yeah Griever I know what you mean about "grinding back up very slowly". Sometimes I drop in the 1950 range and it takes FOREVER to get back to 2000. It seems like the 1900 level problems are on average harder than the 2000 level problems. It's weird.
Yeah Griever I know what you mean about "grinding back up very slowly". Sometimes I drop in the 1950 range and it takes FOREVER to get back to 2000. It seems like the 1900 level problems are on average harder than the 2000 level problems. It's weird.
I had a similar feeling.
When I finally made it to 1500, the problems seemed easier. When I got to 1550, all of a sudden, they seemed harder.
I heard someone once say it is probably more about problems that have found my weakness, rather than being inheriently more difficult.
I've decided it is better to agree with this assessment and pay more attention to problems I miss and why I may have missed them.
I remember several years ago my tactics trainer rating was 2200+ but now I struggle to stay above 2000 even though I'm sure I've gotten better. I usually only get a few points for a correct answer and lose at least twice as many when I get one wrong. Also a lot of the time I'll see that a problem used to be much higher rated. Is that because more and more people learn the problem and therefore bring the problem's rating down over time?