For an athlete, no matter what level they're at, there will come a time where your performances will not seem to be going anywhere. When you're relatively young in the sport and taking it pretty seriously those periods will be less frequent, but as you get older and more experienced that point will come, and that's the point that defines you as an athlete.
Analyse your performance
The first step is to realise when your performances have don't ignore it for too long, analyse and reassess what's happening. Most people are knowledgeable enough about the sport they invest so much in that they can pretty much self diagnose the reasons behind their poor performance. I'd say go with your gut, be honest with yourself and make some changes, but don't make irrational decisions.
Undertrained? - prepare harder
Perseverance is also important. Perhaps it can be that you're undertrained and the answer for that is simple...choose to invest more in preparation time limiting the amount of distractions, which could include cutting back on racing to get ready for a strong performance later down the line. It takes more than a week or two to make such changes.
Overtrained? - then rest and return refreshed
Triathlon is a draining sport and in that case it could be that you're getting tired, or emotionally trained, perhaps over trained but more likely as an amateur just trying to balance too much with work, training, lack of sleep and time for yourself. In this case, take a rest, it will do wonders and it actually takes longer than 10 days to lose your base. You may have the feeling that when you come back you're under trained but fresher and more motivated. I always say better to be underdone but fresh then fit and tired. Train smart!
Pay attention to the small details
Triathlon is a competitive sport, at the top age group level they're very good athletes now. So you'll find that each year you have to raise your game to keep similar results. Keep an eye on the small things that will make you faster. Bike position, equipment choices, technique, tactics, all those things that will leave you behind if you neglect it. Sometimes the answer isn't extra training.
In cases where you can't quite put the finger on what's going on it may be worth getting some blood work done. I use a company called Indurance. I get some blood drawn and send it off to them and get a full profile and analysis back from them either identifying problems or offering peace of mind that it's not my health, it's something else. However outside of that keep an eye on your general health, get enough sleep, eat good quality balanced diet and that will always help you.
Finally have a word with your coach, if you have one. Change is as good as a rest. Each coach has their own methodology and undoubtedly you can't fit everything into your programme. So likelihood is that with another coach you'll increase your artillery and motivation, gain experience which is invaluable as an athlete.