Graham Brown, from Canaccord Genuity, said the Scottish company has been slow to convert prospects into concrete sales as well as facing price and currency pressures.
Mr Brown, who issued a note to begin coverage of Aggreko, also pointed out a number of large contracts, in the military arena and in Japan, are expected to come to an end across 2013.
It was also suggested that rival APR Energy appears to be making inroads into Aggreko's market leading position.
As a result of all those factors, Canaccord is forecasting Aggreko's results over 2014 and 2015 will be at the lower end of market expectations.
Mr Brown said: "Both consensus estimates and the shares have been under pressure since Aggreko cautioned on the conclusion of material (Japanese & military) contracts in 2013, together with tougher conditions in its emerging economy customer base, almost a year ago.
"The continuation of these issues, plus more recent adverse currency movements, does not appear to be fully factored into consensus.
"[Aggreko's] competitors are the network of Caterpillar dealers and APR Energy. Data on the market share of the former is elusive, but APR Energy has seen strong order momentum and seems to be gaining share."
Analysts expect Aggreko to report pre-tax profit of around £340 million for 2013 as new power projects in Africa come into operation.
Profits for the first six months of the year came in at £146m on revenue of £745m.