A market could be difficult to trade at high levels due to magnified volatility in terms of it's absolute value.
A 1% move at 5,000 (whatever unit it is) is 50 ticks while a 1% move at 15,000 is 150 ticks.
In leveraged trade, a trader could be right in predicting the market direction, but he may not be able to withstand the volatility in a 150-tick range while a 50-tick range volatility is easy for him to absorb and handle.
At high levels even when market is trending it could be very volatile in short-term and hence shake out both leveraged longs and shorts.
If one is good at spotting price trends, go for markets that are trading at relatively low levels and add to positions all the way up or down.