I think due interest rates and lack of full master bedroom we can’t get it off the market .Any advice on what we can try? 

What is their nightly rates and occupancy?

Valid reasons might include increased operating costs, property improvements, or market trends.Fairness and market rates: Rent increases should generally be reasonable and in line with market rates for similar properties in the area.

Interest rates change, regulations change, in CA lots of fights happen, inflation happens, and just so many headwinds to fight thru as a developer. 

Code § 7701 - Definitions, my understanding is that the answer is NO, but maybe I am wrong.As the actual rate of withholding is 15% of the sales price, and as the refund (if any) would be "within 90 days", it can be a major setback for my company´s flipping cash-flow because 15% is way higher than the taxable income (and of course than the tax itself), so we would be in the necessity of capital injections to flip the next properties.If any of you have comments on this, it would be very helpful.Thanks in advance

My question is, would adding a shed to these properties increase rental rates?

I would do 5% (ish) increases year over year until I got to the desired rate. 

For wholesale purposes you will find distressed properties that when fixed are still in affordable price ranges for end buyers especially with where rates are, or work on DSCR products.

Specially at this current interest rate.

Now if you have a place that is short term say in summer and mid-term winter because you get a significantly higher rate in one season for STR that makes sense.