I would take a look at these two links in depth -
1.
https://www.canada.ca/en/immigration-refugees-citizenship/services/immigrate-canada/express-entry/documents/proof-funds.html
2.
https://www.canada.ca/en/immigration-refugees-citizenship/services/immigrate-canada/express-entry/documents/offer-employment.html
Did you submit anything else along with the job offer letters? There appears to be quite a bit more you have to submit if you are to be exempt from having to provide Proof of Funds under FSW. Further, at the time of making your application, you need to be authorised to work in Canada (on a current temporary work or study permit).
Have they requested an additional document or attempted to communicate at all?
You can use a gift deed to justify a sudden transfer of funds (bank statement and also letter from the financial institution, see link one) however it will be at the discretion of the officer assessing your application as to whether they accept the transfer of funds and gift deed. The reason I say this is that they are meant to assess the application at the time of submission, not what you can prove now. The only other way I could think of is if you are able to quickly get the additional documents listed on the second link and upload them along with a letter of explanation.
Good luck.