thats your problem right there, temperature, you want to have your incubation temperature set to a minimum of 27 and no higher than 29 c, any colder and they will take a lot longer to grow (what happened to you) and much hotter and you could kill your mycelium.
so you might want to make an incubation chamber, which is 2 big plastic totes inside each other, the bottom one filled with water, with an aquarium heater in the bottom one, aquarium heaters allow you to set your temperature exactly to the degree, so set it at 27c / 80f (the mycelium makes its own heat when growing so this is high enough).... using a incubation chamber you should be able to fully colonise your cakes in 2 weeks ish, rather than 5!
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hrm, thats not the strain (B+ in your case), they are all cubensis, they say a cube is a cube is a cube...
Your problem is more likely to be that your B+ spores / syringe where older or stored badly, and / or your temperature was out (usually to low) .... old / badly stored spores will make the resulting mycelium grow considerably slower and low temps stunt growth.