I don't think that is the problem, they will be in competition for water and nutrients. they are 'hungry' plants so the lobelia will be taking some of their nutrients.'
so feed and water mixed plantings more.
hope this helps.

Well I know begonias like dappled light (under a tree is ideal) They are also very hungry and very thirsty plants - they are in constant bloom. They want rich dark compost that's kept constantly moist and warm temperatures. Don't ever let the pots dry out. As SBG has stated, they will deplete the nutrients in a pot very quickly so they should be fed about every 2 weeks (fish fertilizer works really well). The lack of anything will interrupt the flower cycle. We call them Prima-Donnas of the garden.

Also, don't overcrowd the pot. I did this and had to transplant the orange one - it never bloomed again. They can't really compete with weeds or other plants. They stress very easily and have a hissy fit if everything isn't just so. Yes, they demand a lot but wow they are gorgeous plants especially when cascade over the sides. My pink & yellow one was covered in giant flowers all summer and big fancy leaves and will continue until frost takes them out. They really earn their keep and if you store the tubers indoors, you can plant them out again next year.

Thank you both