I have two Noritz tankless water heaters which were serviced/flushed recently. I often have trouble getting hot water out of my faucets/showers. Sometimes it just takes a long time, sometimes it just doesn't come. One thing that seems to work at multiple locations is turning off the cold at the angle valve, but of course I can't do that for showers and some of the faucets. Once I get the hot water, I can turn the cold back on and I'll still have hot water. Water heaters are firing up when I turn on the hot water, and the pipes coming out of the water heaters are hot, so I don't think it's the water heaters. I do have recirculating pumps, but the problem is there whether or not they are running. Any ideas what could be causing this or techniques to troubleshoot?

Why do you have a recirculation system and tankless heaters? The tankless rely on sufficient demand from the hot side to activate. If you already have some hot/warm water in the system while it is running, then it may not be pulling enough demand to fire the heaters. If the recirc is off, I suspect you have a bad check valve and it is drawing some cold backwards and not enough hot demand for the heaters to fire (or you have a cross connection from a single handle valve) If shutting the stop off on a cold supply works to get hot water, leave it off and try showering. There may be a cross connection in the single handle sink that is affecting everything. It is probably going to take someone with a infrared thermometer and a lot of diagnostic work on site to track this one down. Out of curiosity, how are your tankless units configured? Does one unit feed the second unit?

The other guy pegged it! You shouldn't have recirc pumps on a "heat on demand" system. Recirc's are only for a hot water tank that can heat and store several gallons of hot water and then the recirc system uses the heater to heat the piping in the house as well. Shut those pumps off and remove them. You also have a cross connection. Did you install this recirc system by piping the cold and hot together under the sink somewhere in the house at the furthest point away from your old water heater? These types of household recirculating systems again are designed for holding tank style water heaters and it's advisable to remove these connections now that you have tankless water heaters. I've not heard of anyone using a recirc system with these types of water heaters as they would heat the water often when the temperature drops in the piping, which defeats the purpose of money savings using a tankless! AND a recirc system can draw water through mixing fixtures such as a shower valve or commercial grade mop sink faucets, etc. which causes a cross connection.

P.S. Insulate all hot water lines in the house to effectively minimize temperature loss in piping from water heater to user from the tankless water heaters.

Insulating ALL the hot water piping in the house is good in theory, but VERY hard to do in practice.

Also out of curiosity; is the house on a well system?

Navien makes models of tankless water heaters with a built in circulator pump if set up properly most tankless water heater can have a recirc hooked up.