If you're slowly increasing but still slightly under market, have a better product, or better service (or all 3) and have rental comps to back it up, if they start to ask you can highlight these points. 

Have you ever thought about selling some (or all) of them with seller financing or doing something creative like the "subject to" method? 

You could then see into the ceiling/floor cavity to investigate where it's coming from, if anywhere. 

However, there is a special allowance under which some or all of your passive activity loss may be allowed.

If you can househack something like a 2-4 unit your down payment money might go further and allow you to borrow more since often times lenders will factor in some or all of the other rent as income in the DTI calculations.

I need a second mortgage to cover most or all of the rest.  

If you are a seller willing to pay portion or all of a buyer’s commission, make sure your  listing agent knows that and is aware of how much.

.- on a fixed /closed end loan you cannot reaccess your funds so if you plug in all of your available excess funds as "extra," principal you could have an emergency where you need to access your funds again and you'll be stuck- on HELOC's and 1st position liens the interest is calculated daily and with regards to products like AIO or all in one mortgage from CMG financial the interest is calculated daily and settled/swept at 12AM each day allowing every outstanding dollar you park into your line to work for you within your line of credit.

This has always been the case, but the industry is taking strides to make it more fluid.Investor Sellers can choose to pay some or all of the buyer's agent commission (that's not new) but this can no longer be advertised through the local MLS AND this must go through the seller to the agent versus through the listing agent like it did in the past.Investor buyers must be signed with an agent to step foot in listed homes.

@Kenny Stevenson If you’re not handy yourself and planning to do some or all of the rehab on a BRRRR it can be an uphill battle for a beginner to accomplish.