You're not sleeping enough. 

Making time for exercise is important –but sacrificing sleep in order to squeeze it in isn't the best solution. Lack of sleep is an acknowledged factor in weight gain possibly because it can disrupt the hormones that help regulate appetite, increasing the urge to eat. 

Those 100 calorie snacks don't fill you up.

Not sure who came up with the   100 calorie snack  concept but  a quick Google results in pages of ideas – like a cup of frozen grapes,  two figs stuffed with ricotta or eight cocoa dusted almonds. All perfectly nutritious but can they fill the stretch between lunch and dinner – or will you still be so hungry you keep picking at food and clocking up extra kilojoules?   

"If you have an appetite that can be satisfied with just 100 calories (400 kilojoules) it may be that you're not really hungry – you just want to snack," says Associate Professor Amanda Salis of the University of Sydney's Boden Institute of Obesity, Nutrition, Exercise and Eating Disorders.  "If you're genuinely hungry you may be better off with a proper meal or a substantial snack like a small toasted sandwich or hommus with veggies that keep you satisfied and prevent grazing."

Your body is fighting back. 

One reason why weight loss can be difficult is because the human body is designed to protect its fat stores in case there's a famine around the corner. If you've succeeded in losing weight but can't shift the last few kilos despite  doing all the right things, you're up against what Dr Salis calls 'the famine reaction' - where your body fights hard to keep the weight on.

"I'd suggest taking a break from trying to lose weight for two to four weeks.  Just try to maintain weight rather than try to lose it - and then start again," she says.

You may be eating more than you think.   

"After a few weeks of trying to lose weight people sometimes get complacent and snack when they're not hungry.  This is where a food diary can help to get a true picture of what you're eating," says Salis. "But rather than just recording what you eat and when, make a note of how hungry you are when eat – and if you're not hungry it's a clear sign you're eating more than you need."

You have PCOS – but don't know it.   

Around one in five women of reproductive age have Polycystic Ovary Syndrome (PCOS) which can cause infertility and make weight loss difficult - but an estimated 70 per cent of cases go undiagnosed.   Exactly why PCOS makes it easier to gain weight but harder to lose it   isn't clear - but suspects include increases in testosterone and insulin levels that can encourage weight gain.  Symptoms like acne, weight gain, excess hair or hair loss and irregular periods can all be signs of PCOS.

Although the emphasis is often mainly on diet to help manage PCOS, regular exercise matters too - not just to help control weight but for reducing insulin resistance which increases the risk of heart disease and type 2 diabetes, says Associate Professor Nigel Stepto from the College of Sport and Exercise Science at Victoria University who's involved in a number of studies to learn more about why PCOS can lead to weight gain and how exercise can help. 

 "Any physical activity including exercise sessions such as walking, cycling or exercise classes or just being active around the house is good but activities focusing on building strength are also important for managing insulin resistance," he says.

But sometimes barriers like poor body image, lack of confidence, anxiety or depression which sometimes go with PCOS make it hard for some women to be more physically active. This is where a GP referral to an Accredited Exercise Physiologist (AEP) and dietitian can help, says Stepto – they can design and support a program geared to managing PCOS through exercise and diet.  

SMH