Duo Heater issues

I bought an apartment last year. Have 2 Duo Heaters in living room and 2 epx panel heaters in bedrooms. I find that they are not heating the property well. Let me give you some info first about apt. Built 2008 and on 3rd floor and fairly well insulated. Living room is 26sqm with 2 Duo Heaters 500i and 300i while both bedrooms are 14sqm with 1.2kw epx dimplex panel heaters. No 4 zone dimplex controller system. My electric bills are also on the high side but i am more interested in having proper heat in apt. Firstly the 2 Duo Heaters. I set the background heat to 5 and i keep the radiant heat at 1. The 500 keeps the heat pretty ok till about 7pm evening time and then i up the radiant heat when i get home from work. The 300 does not really heat up at all. Its just luke warm. I thought the 500 might be effecting it but i switched off the 500 one night and tried the 300 on its own and same results. The issue i have with the panel heaters in the bedrooms is that they take ages to heat up the rooms. I had old type storage heaters in last property and plx panel heaters and the place was cosy all the time.

First thing i want to know from anyone with experience is do they think there is issues with the heaters or are they not sized correctly for the property? I am asking this as most people who repair storage heaters in the area also sell the likes of farho, roint , lucht electric heaters and i want as much info as possible before i get someone in.

I also have a Dyson heater to help boost the heat but now that has been recalled:(:(:(