The maximum power that the inverter can supply is what we used to call as Surge, but it is usually for only a short time - may be for a few seconds up to 15 minutes. For few electrical appliances, especially with electric motors will be needed much higher surge. even Pumps could be considered along with the refrigerators (compressors) as an examples.
In Your case:
Check for the actual surge rating of your 2kva inverter. I have considered 2kva is the surge rating, which means it can carry 2kva for around 15 seconds before it gets trips off.
Photostat machine may be rated with 1800 watts and the surge could be even more. That means when started using the Photostat machine, it will use up more than 1800W[surge] for a few seconds then drops to 1800W[continuous] for steady operation.
So which is leading to the flickering problem. So Increase the surge rating will not serve you as a permanent solution. So its better to use another inverter with higher surge rating. Generally Inverters are available in different Size ratings from 50va to 50kva. Inverters larger than 11kva are not often used for household applications.
P.S. Different brands of inverters have different surge capacity