Yes, it is related to the wetting of your solution with the substrate. Changing the concentration of the solution or spinning parameters won't help.

If you don't want to change the solution, try firstly to find in the literature (I do not have experience with glass substrates and polymeric solutions) if there is any solvent that might be used to treat the substrate prior to deposition. What I mean is that changing characteristic of substrate can help.

The surface of the glass may be too smooth and hydrophobic. Try do do some treatment to make the glass surface to become rough.

May be the solution also a factor that cause your solution spin out from your glass substrate. Can try to use the solvent that is hydrophilic to the glass substrate.  

It sounds from your description your PLA/chloroform solution is not wetting the surface of your substrate. Thus first you should be sure that your glass substrate is scrupulously clean. I suggest you use a piranha solution (H2SO4)+hydrogen peroxide to clean your substrate. Check web for details. As you know PLA has a high surface energy ( a drop of water on PLA has a contact angle around 60 Deg)., so any chance it has to de-wet from the glass, it will. To make matters worse, Chloroform, although a great solvent, its downside in spin coating is its high vapor pressure, so if you have non -uniform evaporation, dry spots can form on the surface and  then PLA my precipitate out of solution,  and with it high surface energy simple get spun off the substrate. Also, try reducing the spin speed see if that helps. Good luck

I found in the literature that adding dimethyloformamide to chloroform-PLA solution could reduce vapor pressure and is used for electrospinnig of PLA fibers. However, adding DMF did not improved poor wettability of glass substrate in my case. Maybe other substance will be better? (For example, adding ethyl alcohol to graphene oxide-water solution allows coating of smooth glass substrate.) Another question is whether it would cause degradation of PLA?  (Thank you for responses!)

dichloromethane could be used as solvent, although it evaporates quick as well, I use 2000rpm for 10% PLA solution on glass slides and it works out pretty good.

what is the molecular weight of your PLA ? In order to have good film-forming properties this should be sufficiently high ! Secondly, cleaning overnight in piranha, as suggested by Brian, rinsing with water and silanization in toluene is the common procedure  to change surface free energy of glass substrates. Do you spincoat on glass-cover slips ? Sometimes good cleaning with desinfectol would also work for these, for other glass substrates the piranha + silanization pre-treatment is best. Furthermore, use relatively low concentrations in chloroform, not more than 3 w%. Alternatively, you could try to work from THF instead of chloroform, it takes longer to dissolve the polymer however. Hope these are some usefull tips, good luck  ! 

First of all you have to clean your substrate with piranha solution, rinse with water,  After that, you should silanize your substrate as Tim suggested and it definetly change your surface free energy.

Your polymer concentratiion should be low (1-3% depending  on your polymer molecular weight) for this kind of applications. You may add some DMF or THF to your solution to prevent precipitation  or  you can also try dichloromethane or THF as Sen and Tim suggested. If you have solubility problem you may use ultrasound for a short time but be careful not to face with degradation problem. Depending on your solvent type choose your spin coat speed. If you use low temp. evaporating solvent like chloroform use low speed.

Good luck

I have heard that long storing piranha solution is not the best idea, but if I have to transport it between buildings, what kind of material do you recommend?  Thanks a lot for recent responses.