It sounds like you don't have either sufficient power generation to meet your fluctuating power demands while also charging your accumulators or enough accumulators to supply enough power overnight. Accumulators will only charge if you've got excess power available in your power distribution network. Think of accumulators as giant batteries that store excess power and release that power when you don't have enough generation capacity.
So if your factory demands 90 MW/s of power to run, and you're producing 120 MW/s of power from your steam engines and solar panels. Lets say it's 80 MW/s from steam and 40 MW/s from solar. The excess of 30 MW/s will be channelled into and charge your accumulators. However at night, when your solar panels stop generating power, then your accumulators will supply 10MW/s of power to keep your factory running.
However each accumulator can provide 300 kW/s and stores 5MJ of energy per accumulator. (1 Joule = 1 Watt per second). According to the linked Wiki page, it takes roughly 17 seconds for a single accumulator to charge/discharge at the max rate of 300 kW/s. I have no idea how long night lasts, but it's significantly longer than 17 seconds. Given the above scenario, you're going to need at least 34 accumulators to provide 10MW/s of power collectively, and they will be able to do that for 17 seconds.
It sounds like you need more accumulators than you currently have so that they last through the night. I like to set up a builder to automatically create accumulators and just forget about it until I need to build more.
I like to build accumulator blocks in the following fashion:
A A A A A A A
A A A A A A A
A A A A A A A
A A A S A A A
A A A A A A A
A A A A A A A
A A A A A A A
A: Accumulator
S: Substation
Where A is an Accumulator and S is a Substation. I've got many many many of these ones built for my factory along with the popular solar panel plant design where you use substations, solar panels, accumulators and robo-ports together to build large tracts of solar panels separated by accumulators.
It's worth noting that if you're doing a large scale operation involving lots of robots (for example using them to lay concrete, or the large scale building of solar plants) the robo-ports will draw a very large amount of power while they are building. I had this happen multiple times where my robots drain my accumulators overnight while clearing large tracts of land. The solution was to place more solar panels and accumulators to store more energy. I'm up to about 3.2 GJ of accumulators I think.
Happy building.
