Basing on personal experience (you can find more on Google probably), solutions are:

On percolated (filter) coffee machines:

Check water temp, if water temp is insufficient , most common is scale heating element , faulty element, faulty thermostat.
Check if water is dripping right at the center of the filter, if it is dripping on one side because the orifice is clogged, that can result in weak coffee.
Coffee blend not well distributed on filter basket.

On Espresso machines:

Check water temp, same as filter coffee machines (thermostat element, scale on heating chamber).
Ensure that coffee is filled correctly in the filter cup (fill at level), and well pressed.
Ensure that the coffee mixture that you are using is not too coarse, buy the apposite one for espresso.
Ensure that you are not leaving the machine brewing for a time that is too long, and Italian coffee cup is very small, approximately one regular shot.

On the hob moka pots (espresso, Moka and Bialetti type):

This are the trickiest ones, you need art and practice to get a good espresso with a Moka, but when you learn, the result is better than what you may get with an home electrical espresso maker.
The frame on the hob must be not too strong, and the mixture coarse but well pressed, with water at level tag, or you will get a weak coffee.
On Mokas a good strategy is keeping hob at minimum, and turning up the flame at top when coffee starts coming out.

French and turkish cafetieres:

Ensure that you put enough coffee, water hot enough, and that you left the infusion enough time to get a strong coffee.

Note: insufficient pressure and clogged lines on electric machines never give you a weak coffee, exactly the opposite.
Not pressed mixture always gives weak coffee, but too much pressed mixture may give you coffee with bad taste.