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On a tankless system the most common problem with not having enough hot water can be the coils clogged with sediment and then the mixing valve that sets the water temp is another common problem. The valves fail clogging with scale and sediment and either jam or stick and sometime they move but are not adjusting the water mix. They are located just outside of the tankless coils on the out (hot) side and normally have a black plastic knob with some increment markings. The mixing valve mixes in cold water to adjust the hot water to the desired temperature. If it is not working properly you will get cool or limited hot water.

You also mentioned that the water pressure was low. Are you on a well or community water sytem? If you are on a well I would also check pressure tank at the pressure guage to see that the pressure is high enough - 30 pounds and more is normal. Community water should have sufficient water pressure. A clogged tankless coil or mixing valve can also reduce your water pressure/volume.

Hope this helps.