There are a number of factors that could cause this problem.
First of all, was it wild caught or captive bred?
A wild caught animal is often stressed and can suffer more if it had any parasites or ailments from its native rainforest home to begin with. The stress of transport and adapting to captive life can be difficult on the animal.
Ask whoever you bought it from. Ask how long they "conditioned" them after being taken if they were WC.
Also, what size tank do you have? What is the humidity, temperature, and does the anuran have a decent timed photoperiod?
Force feeding can be extremely difficult. I am no expert on force feeding, as I have never tried to force any anuran to do this, as they usually end up dying from some ailment, not just captive stress. There is no one surefire method either.
Before you try and attempt this, make sure first everything in the frog's environment is correct. Sometimes, sometimes that is, liquid vitamin sprays for reptiles can be useful. Some actually help stressed animals, but these are mainly for reptiles and not amphibians. But water soluble vitamins would easily pass into a frogs skin. I did this once on an African Clawed frog that I had a long time ago, and I can assure you that it will not kill them or make them sicker. But not every spray is the same, so use your intuition to find the best one, as I cannot say (mine was discontinued). This frog I put to sleep because he had Cancer and tumors.
Spray the frog, after carefully reading what is in the vitamins first. Water soluble is best, like vitamin C and electrolytes like potassium and sodium. Vitamins like A are fat soluble, and useless.
Do not stress her out anymore. Best done while the animal is sleeping during the day.
Parasites are very difficult to treat. Many wildcaught frogs can harbor nemetodes, parasitic worms. Luckily, there are medications to treat them. Ivermectin is best for Rhabdias nemetodes. They can be seen crawling on excretement, extremely tiny white things. Rapid weight loss is common. Ask you veternarian for Ivermectin if you suspect this. Drip or soak the frog in this solution. Misting is another option. Or an injection, I have done this and it is not easy for the frogs or the vet.
As a final resort, force feeding supposedly is done by getting a tongue depressor. Hold the frog firmly so it cannot jump. Best use latex gloves so the oils in your hands will not cause more harm. Freeze a few crickets to kill them and soak them in a liquid vitamin spray for a while. Once you pry the animals mouth open, push it in. You can also administer medicine this way.
Depending on the frog, he may or may not make it. But good luck to you!
pitcherplant7
2 clawed frogs
2 firebellied toads
1 tinctorius dart frog